qt4: Don't create QMenu without parents

qt4: Don't create QMenu without parents

This patch ensures that _all_ menus/actions have got a parent widget that
is guaranteed to be deleted.

This patch
   - solves memory leaks for menus and action(children)
   - solves a side effect (crash) pointed out
     by 193e6eacddb63e8a5bda42395ee79b5b27db6408 where
     some vlc objects were no longer properly released for lack
     of a clean menus/actions release chain.