Draft: variables: add var_AddCallbackChecked,var_AddListCallbackChecked

With variables set now on vlc_player, modules now need to rely on var_Inherit() or have to walk the parent objects to place a callback.

Current API we have no way to tell if a registration fails and we can't register callbacks with top level objects without iterating over parent object using GetChecked() at each level.

Note: Updating/Replacing the current AddCallback API is also an option