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/ViewProviderTemplate.cpp

@@ -195,6 +195,10 @@ MDIViewPage* ViewProviderTemplate::getMDIViewPage(void)
     return myMdi;
 }
 
+Gui::MDIView *ViewProviderTemplate::getMDIView() {
+    return getMDIViewPage();
+}
+
 TechDraw::DrawTemplate* ViewProviderTemplate::getTemplate() const
 {
     return dynamic_cast<TechDraw::DrawTemplate*>(pcObject);