CppEditor: refactor FollowSymbol

Create an interface to get the ability to use
another FollowSymbol implementation

Change-Id: I5802f62523ff3ee47b8a14e487adf43edcb6c9b1
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/cppmodelmanagersupportinternal.h

@@ -43,6 +43,7 @@ public:
     virtual CppCompletionAssistProvider *completionAssistProvider();
     virtual BaseEditorDocumentProcessor *editorDocumentProcessor(
                 TextEditor::TextDocument *baseTextDocument);
+    FollowSymbolInterface *followSymbolInterface() override;
 
 private:
     QScopedPointer<CppCompletionAssistProvider> m_completionAssistProvider;