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/libs/modelinglib/qmt/document_controller/documentcontroller.h

@@ -61,6 +61,7 @@ public:
 
 signals:
     void changed();
+    void modificationChanged(bool modified);
     void modelClipboardChanged(bool isEmpty);
     void diagramClipboardChanged(bool isEmpty);