TextEditor: Use callback in refactoring markers

Allows to trigger actions without adding specific handling into the
editor.

Change-Id: Ia63d65d3feca37bcefca1b6322ade039027a92d8
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/cppeditor/cppfunctiondecldeflink.h

@@ -70,8 +70,6 @@ class FunctionDeclDefLink
     Q_DISABLE_COPY(FunctionDeclDefLink)
     FunctionDeclDefLink() = default;
 public:
-    class Marker {};
-
     bool isValid() const;
     bool isMarkerVisible() const;
 
@@ -113,5 +111,3 @@ private:
 
 } // namespace Internal
 } // namespace CppEditor
-
-Q_DECLARE_METATYPE(CppEditor::Internal::FunctionDeclDefLink::Marker)