Cocoa: application menu items sometimes get duplicated

Cocoa: application menu items sometimes get duplicated

Under some circumstances, the same menu item appears several times
in the application menu in the menu bar. This can be seen in Qt creator,
where "About Qt Creator" appears twize.

The reason is that QCocoaMenu::syncMenuItem does not take into account
that merged items cannot be found in the QCocoaMenu that owns the
menuItem, but rather inside the application menu. And because of this,
it fails cleaning up the old item when it changes from e.g
TextHeuristicRole to ApplicationRole.

This patch will fix this.

Change-Id: Ia84f552d1788d80d778c7dded3393412b9d2d8cb
Reviewed-by: Chris Meyer <cmeyer1969@gmail.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>