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.cpp

@@ -643,6 +643,14 @@ bool ScxmlDocument::changed() const
     return !m_undoStack->isClean();
 }
 
+void ScxmlDocument::setChanged(bool modified)
+{
+    if (modified)
+        ; // we lack a setDirty method in QUndoStack
+    else
+        m_undoStack->setClean();
+}
+
 ScxmlTag *ScxmlDocument::scxmlRootTag() const
 {
     ScxmlTag *tag = rootTag();