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/DrawViewDraft.h

@@ -64,9 +64,6 @@ public:
 
     virtual short mustExecute() const override;
 
-    void Restore(Base::XMLReader &reader) override;
-
-
 protected:
 /*    virtual void onChanged(const App::Property* prop) override;*/
     Base::BoundBox3d bbox;