Replace virtual isModified method with a getter/setter/notifier

We have lacked the setter and dedicated notifier before.

Change-Id: I58845a48259d260c5cc90ae94b173c79cddcfef9
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/scxmleditor/plugin_interface/scxmldocument.h

@@ -156,6 +156,7 @@ public:
      * @return - true if changed, false otherwise
      */
     bool changed() const;
+    void setChanged(bool modified);
 
     /**
      * @brief rootTag - return rootTag of the document
@@ -277,6 +278,7 @@ private:
     ScxmlTag *createScxmlTag();
     QString m_fileName;
     QUndoStack *m_undoStack;
+    int m_cleanIndex;
     QVector<ScxmlTag*> m_tags;
     QHash<QString, int> m_nextIdHash;
     QHash<QString, QString> m_idMap;