Expose invokables that are not slots in the xml

Without it the invocations were working but were not listed on introspection

Backported from Qt5: qtbase commit c3f485c5250a503832e767e1fe5e40595126f6c5

Change-Id: Ie62f7dc3577f52b6888ddebf0392fdf51f2845a5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>