Introduce IBlock and ILettersBlock interfaces (Round 2) (#1263)

* Code review changes
- Keep the Bounds property on the image classes so this isn't a major breaking API change
- Don't expose letters collection

* Minor fix

* Switch to using BoundingBox in the library

---------

Co-authored-by: davmarksman <david@brokit.co.uk>

src/UglyToad.PdfPig.DocumentLayoutAnalysis/TextBlock.cs

@@ -9,7 +9,7 @@
     /// <summary>
     /// A block of text.
     /// </summary>
-    public class TextBlock
+    public class TextBlock: IBoundingBox
     {
         /// <summary>
         /// The separator used between lines in the block.