App: add new convenient class SubObjectT

Derived from App::DocumentObjectT to add support of sub object

Also some minor changes to App::DocumentObjectT

Changed Gui::SelectionChanges to make use of SubObjectT

src/Mod/Sketcher/Gui/ViewProviderSketch.cpp

@@ -1485,7 +1485,7 @@ void ViewProviderSketch::onSelectionChanged(const Gui::SelectionChanges& msg)
     // are we in edit?
     if (edit) {
         // ignore external object
-        if(msg.ObjName.size() && msg.DocName.size() && msg.DocName!=getObject()->getDocument()->getName())
+        if(msg.Object.getObjectName().size() && msg.Object.getDocument()!=getObject()->getDocument())
             return;
 
         bool handled=false;