Merge ITextMark and BaseTextMark

Turns out we don't even need two-phase initialization,
by transparently postponing registration until we get
a non-empty file name, either at constuction, or at
file name change times.

Change-Id: I3e87e47c820066e6707e946fc474ab9c1993e61f
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/cpptools/cpptoolseditorsupport.h

@@ -51,7 +51,7 @@ class DeclarationAST;
 
 namespace TextEditor {
 class BaseTextEditor;
-class ITextMark;
+class TextMark;
 } // namespace TextEditor
 
 namespace CppTools {