Derived from App::DocumentObjectT to add support of sub object
Also some minor changes to App::DocumentObjectT
Changed Gui::SelectionChanges to make use of SubObjectT
* Add new API and signal handler in document observer
* Pre initialize python handler function to improve performance. In
case Python code use dynamic patching, i.e. add class method at
runtime (which is rare and should be discouraged), the python feature
can be re-initialized by simply assign proeprty Proxy again.
* Add property tracking in DocumentObjectT
* WidgetFactory adds support for accepting python QIcon, which is used
by ViewProviderPythonFeature
