Minor QPlatformMenu API cleanup.

Minor QPlatformMenu API cleanup.

Make the QPlatformMenu* classes abstract with
pure virtual functions. Delete qplatformmenu_qpa.cpp.
QtGui no longer has to carry the empty implementations.

Change the inheritance tree - Make QPlatformMenu
and QPlatformMenuBar inherit QObject instead of
QMenuItem. Add (now) missing virtual functions to
QPlatformMenu. The effect of this change is to
reduce the number of pure virtuals the platform
has to implement.

Change QPlatformMenuBar::syncMenu to take a
QPlatformMenu pointer instead of a QPlatformMenuItem*
pointer.

Fix a couple of header/unused variable
compiler warnings.

Change-Id: I77450e40c6aae26f3e1aca204603acebd242c036
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>