controller: move to core/gstobject

controller: move to core/gstobject

Move the controller to gstobject as a simple delegate. The controller and
controlsource are not classes in core. The controlsources stay separate as a lib
for now. This way we can avoid the qdata lookups.

Also remove controller_init(). There is no more need to link to controller for
elements.

Also sanitize the API. We now have functions to add properties like we had
methods to remove that. That avoids then ref count hacks we had in _new.