Modify markup tag interfaces and fix some bugs

[ElectroJr]

This PR marks the IMarkupTag interface and associated methods as obsolete, and adds a new IMarkupTagHandler interface and methods in MarkupTagManager that should replace the old ones.

The main reason for this change is to try prevent confusion & bugs, due to users assuming that there are multiple tag instances per rich text tag, when in reality the tag manager only ever creates one instance per tag. This confusion has already lead to a bug in content, where TextLinkTag assumes that it has an instance per control.

This PR also fixes another bug, where controls that were using the RichTextEntry constructor to add child controls never removed those controls once the entry was removed.