Add basic rich-text support to AvalonEdit.
Introduce 'RichText' class and use it in new output pad API.
Replace HighlightedInlineBuilder with RichTextModel.
Fix RichTextModel.UpdateOffsets()
Set e.Handled = true in SharpTreeView.OnKeyDown()
ModelCollectionEvent: allow removing event handlers from within an event handler
Use NullSafeSimpleModelCollection where possible
Move ModelCollectionChangedEvent to its own file.
Make RichTextWriter and derived classes internal; they're not necessary for using RichText/RichTextModel anymore.

If no new usecase comes up, these classes can be deleted (except maybe for HtmlRichTextWriter).