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/App/DrawProjGroup.cpp

@@ -174,7 +174,10 @@ App::DocumentObjectExecReturn *DrawProjGroup::execute(void)
 //    }
 
     for (auto& item: getViewsAsDPGI()) {
+        bool touched = item->isTouched();
         item->autoPosition();
+        if(!touched)
+            item->purgeTouched();
     }
 
     return DrawViewCollection::execute();
@@ -427,7 +430,6 @@ App::DocumentObject * DrawProjGroup::addProjection(const char *viewProjType)
                 Anchor.purgeTouched();
                 view->LockPosition.setValue(true);  //lock "Front" position within DPG (note not Page!).
                 view->LockPosition.setStatus(App::Property::ReadOnly,true); //Front should stay locked.
-                App::GetApplication().signalChangePropertyEditor(view->LockPosition);
                 view->LockPosition.purgeTouched();
                 requestPaint();   //make sure the group object is on the Gui page
             }