Remove ITextMarkable

There was only a single user (TextEditor::DocumentMarker) which
in turn was only used in a single place (BaseTextDocument), so
merge the ITextMarkable into BaseTextDocument.

Change-Id: I1ca8675f9fc22cb56097b843dc5a0fe76875d3e7
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/texteditor/basetexteditor.h

@@ -67,8 +67,6 @@ namespace Internal {
     typedef QString (QString::*TransformationMethod)() const;
 }
 
-class ITextMarkable;
-
 class BaseTextEditor;
 class FontSettings;
 class BehaviorSettings;