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/ViewProviderGeomHatch.cpp

@@ -202,3 +202,11 @@ TechDraw::DrawGeomHatch* ViewProviderGeomHatch::getViewObject() const
 {
     return dynamic_cast<TechDraw::DrawGeomHatch*>(pcObject);
 }
+
+Gui::MDIView *ViewProviderGeomHatch::getMDIView() {
+    auto obj = getViewObject();
+    if(!obj) return 0;
+    auto vp = Gui::Application::Instance->getViewProvider(obj->getSourceView());
+    if(!vp) return 0;
+    return vp->getMDIView();
+}