TechDraw: Link related changes

* Support link and group objects

* Support view sync by implementing view provider API getMDIView()

* Use handleChangedPropertyType() for object migration instead of
  reimplementing Restore() because of a lots of changes in
  PropertyContainer::Restore().

* Various other small fixes.

src/Mod/TechDraw/Gui/QGIProjGroup.cpp

@@ -84,7 +84,7 @@ bool QGIProjGroup::sceneEventFilter(QGraphicsItem* watched, QEvent *event)
             switch(event->type()) {
               case QEvent::GraphicsSceneMousePress:
                   // TODO - Perhaps just pass the mouse event on to the anchor somehow?
-                  if (scene()) {
+                  if (scene() && !qAnchor->isSelected()) {
                       scene()->clearSelection();
                       qAnchor->setSelected(true);
                   }