Fixing auto-merge

This changeset unifies the two separate TokenFilter implementations (one for BodyPart, TextField, etc and another one for certain elements such as Html).
It also fixes a bug with the TokenFilter when executing for HtmlWidget, where the wrong content item is being recorded by the handler (the original implementation).
Thirdly, it removes awkward code repetition by moving filter execution into a dedicated HtmlFilterRunner service.

src/Orchard.Web/Core/Common/Drivers/TextFieldDriver.cs

@@ -1,6 +1,4 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
 using System.Web;
 using Orchard.ContentManagement;
 using Orchard.ContentManagement.Drivers;
@@ -74,12 +72,12 @@ namespace Orchard.Core.Common.Drivers {
 
                 if (settings.MaxLength > 0) {
 
-                    var value = new HtmlString(_htmlFilters.Aggregate(field.Value, (text, filter) => filter.ProcessContent(text, settings.Flavor)))
+                    var value = new HtmlString(_htmlFilterProcessor.ProcessFilters(field.Value, settings.Flavor, part))
                         .ToString().RemoveTags();
 
                     if (value.Length > settings.MaxLength) {
                         updater.AddModelError("Text", T("The maximum allowed length for the field {0} is {1}", T(field.DisplayName), settings.MaxLength));
-                    }                    
+                    }
                 }
             }