Convert QPA menus to use QIcon, and enable icons.

Convert QPA menus to use QIcon, and enable icons.

Icon support was disabled in QPA menus, now QIcon is in QtGui, convert the QPA interface, enable setting the icon in QMenu, and make the Cocoa implementation use it. (And fix a ref-counting leak if an icon is actually set)

Change-Id: Ica203bf6826b79d8beee58f39febc851b9633a66
Reviewed-by: Christoph Schleifenbaum <christoph.schleifenbaum@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>