Only update the z-indices that do change after reordering nodes

Right now, when updating the z-index of a node in a container,
the indices of all its siblings are refreshed. However, if the
node changes its z-index from z1 to z2, we only need to refresh
the z-indexes of the node's parent's children in the closed
range [ min(z1,z2), max(z1,z2) ]. For groups and layers with
many children that will be more optimal in many cases.

In addition to the above change, this patch:
* forces the parameter of Node#setZIndex() to be in the valid
  range.
* adds the (now absent) unit tests for the methods of Node that
  change z-indices of the parent's children.

test/unit/Node-test.js

@@ -3479,3 +3479,208 @@ suite('Node', function() {
     assert.equal(rect.findAncestor(), null, 'return null if no selector');
   });
 });
+
+test('moveToTop() properly changes z-indices of the node and its siblings', function() {
+  var stage = addStage();
+  var layer = new Konva.Layer();
+  stage.add(layer);
+
+  var rect1 = new Konva.Rect();
+  var rect2 = new Konva.Rect();
+  var rect3 = new Konva.Rect();
+  var rect4 = new Konva.Rect();
+  layer.add(rect1, rect2, rect3, rect4);
+
+  assert.equal(rect1.getZIndex(), 0);
+  assert.equal(rect2.getZIndex(), 1);
+  assert.equal(rect3.getZIndex(), 2);
+  assert.equal(rect4.getZIndex(), 3);
+
+  rect2.moveToTop();
+
+  assert.equal(rect1.getZIndex(), 0);
+  assert.equal(rect3.getZIndex(), 1);
+  assert.equal(rect4.getZIndex(), 2);
+  assert.equal(rect2.getZIndex(), 3);
+
+  rect1.moveToTop();
+
+  assert.equal(rect3.getZIndex(), 0);
+  assert.equal(rect4.getZIndex(), 1);
+  assert.equal(rect2.getZIndex(), 2);
+  assert.equal(rect1.getZIndex(), 3);
+});
+
+test('moveToBottom() properly changes z-indices of the node and its siblings', function() {
+  var stage = addStage();
+  var layer = new Konva.Layer();
+  stage.add(layer);
+
+  var rect1 = new Konva.Rect();
+  var rect2 = new Konva.Rect();
+  var rect3 = new Konva.Rect();
+  var rect4 = new Konva.Rect();
+  layer.add(rect1, rect2, rect3, rect4);
+
+  assert.equal(rect1.getZIndex(), 0);
+  assert.equal(rect2.getZIndex(), 1);
+  assert.equal(rect3.getZIndex(), 2);
+  assert.equal(rect4.getZIndex(), 3);
+
+  rect3.moveToBottom();
+
+  assert.equal(rect3.getZIndex(), 0);
+  assert.equal(rect1.getZIndex(), 1);
+  assert.equal(rect2.getZIndex(), 2);
+  assert.equal(rect4.getZIndex(), 3);
+
+  rect4.moveToBottom();
+
+  assert.equal(rect4.getZIndex(), 0);
+  assert.equal(rect3.getZIndex(), 1);
+  assert.equal(rect1.getZIndex(), 2);
+  assert.equal(rect2.getZIndex(), 3);
+});
+
+test('moveUp() properly changes z-indices of the node and its siblings', function() {
+  var stage = addStage();
+  var layer = new Konva.Layer();
+  stage.add(layer);
+
+  var rect1 = new Konva.Rect();
+  var rect2 = new Konva.Rect();
+  var rect3 = new Konva.Rect();
+  var rect4 = new Konva.Rect();
+  layer.add(rect1, rect2, rect3, rect4);
+
+  assert.equal(rect1.getZIndex(), 0);
+  assert.equal(rect2.getZIndex(), 1);
+  assert.equal(rect3.getZIndex(), 2);
+  assert.equal(rect4.getZIndex(), 3);
+
+  rect1.moveUp();
+
+  assert.equal(rect2.getZIndex(), 0);
+  assert.equal(rect1.getZIndex(), 1);
+  assert.equal(rect3.getZIndex(), 2);
+  assert.equal(rect4.getZIndex(), 3);
+
+  rect3.moveUp();
+
+  assert.equal(rect2.getZIndex(), 0);
+  assert.equal(rect1.getZIndex(), 1);
+  assert.equal(rect4.getZIndex(), 2);
+  assert.equal(rect3.getZIndex(), 3);
+});
+
+test('moveDown() properly changes z-indices of the node and its siblings', function() {
+  var stage = addStage();
+  var layer = new Konva.Layer();
+  stage.add(layer);
+
+  var rect1 = new Konva.Rect();
+  var rect2 = new Konva.Rect();
+  var rect3 = new Konva.Rect();
+  var rect4 = new Konva.Rect();
+  layer.add(rect1, rect2, rect3, rect4);
+
+  assert.equal(rect1.getZIndex(), 0);
+  assert.equal(rect2.getZIndex(), 1);
+  assert.equal(rect3.getZIndex(), 2);
+  assert.equal(rect4.getZIndex(), 3);
+
+  rect4.moveDown();
+
+  assert.equal(rect1.getZIndex(), 0);
+  assert.equal(rect2.getZIndex(), 1);
+  assert.equal(rect4.getZIndex(), 2);
+  assert.equal(rect3.getZIndex(), 3);
+
+  rect2.moveDown();
+
+  assert.equal(rect2.getZIndex(), 0);
+  assert.equal(rect1.getZIndex(), 1);
+  assert.equal(rect4.getZIndex(), 2);
+  assert.equal(rect3.getZIndex(), 3);
+});
+
+test('setZIndex() properly changes z-indices of the node and its siblings', function() {
+  var stage = addStage();
+  var layer = new Konva.Layer();
+  stage.add(layer);
+
+  var rect1 = new Konva.Rect();
+  var rect2 = new Konva.Rect();
+  var rect3 = new Konva.Rect();
+  var rect4 = new Konva.Rect();
+  layer.add(rect1, rect2, rect3, rect4);
+
+  assert.equal(rect1.getZIndex(), 0);
+  assert.equal(rect2.getZIndex(), 1);
+  assert.equal(rect3.getZIndex(), 2);
+  assert.equal(rect4.getZIndex(), 3);
+
+  rect1.setZIndex(2);
+
+  assert.equal(rect2.getZIndex(), 0);
+  assert.equal(rect3.getZIndex(), 1);
+  assert.equal(rect1.getZIndex(), 2);
+  assert.equal(rect4.getZIndex(), 3);
+
+  rect2.setZIndex(3);
+
+  assert.equal(rect3.getZIndex(), 0);
+  assert.equal(rect1.getZIndex(), 1);
+  assert.equal(rect4.getZIndex(), 2);
+  assert.equal(rect2.getZIndex(), 3);
+
+  rect2.setZIndex(1);
+
+  assert.equal(rect3.getZIndex(), 0);
+  assert.equal(rect2.getZIndex(), 1);
+  assert.equal(rect1.getZIndex(), 2);
+  assert.equal(rect4.getZIndex(), 3);
+
+  rect4.setZIndex(0);
+
+  assert.equal(rect4.getZIndex(), 0);
+  assert.equal(rect3.getZIndex(), 1);
+  assert.equal(rect2.getZIndex(), 2);
+  assert.equal(rect1.getZIndex(), 3);
+});
+
+test('remove() removes the node and properly changes z-indices of its siblings', function() {
+  var stage = addStage();
+  var layer = new Konva.Layer();
+  stage.add(layer);
+
+  var rect1 = new Konva.Rect();
+  var rect2 = new Konva.Rect();
+  var rect3 = new Konva.Rect();
+  var rect4 = new Konva.Rect();
+  layer.add(rect1, rect2, rect3, rect4);
+
+  assert.equal(layer.getChildren().length, 4);
+  assert.equal(rect1.getZIndex(), 0);
+  assert.equal(rect2.getZIndex(), 1);
+  assert.equal(rect3.getZIndex(), 2);
+  assert.equal(rect4.getZIndex(), 3);
+
+  rect4.remove();
+
+  assert.equal(layer.getChildren().length, 3);
+  assert.equal(rect1.getZIndex(), 0);
+  assert.equal(rect2.getZIndex(), 1);
+  assert.equal(rect3.getZIndex(), 2);
+
+  rect2.remove();
+
+  assert.equal(layer.getChildren().length, 2);
+  assert.equal(rect1.getZIndex(), 0);
+  assert.equal(rect3.getZIndex(), 1);
+
+  rect1.remove();
+
+  assert.equal(layer.getChildren().length, 1);
+  assert.equal(rect3.getZIndex(), 0);
+});