WindowMenuModel: insert action groups on the toplevel menus
Inserting them on the menu items was a workaround for the long-fixed cloaking-bug in libindicator. It's not supported in gtk 3.10 anymore, because the menu tracker in gtk doesn't allow action groups on menu items. That's for good reasons: it's more efficient and consumers of that api should not modify the created menu hierarchy.
Approved by PS Jenkins bot, Sebastien Bacher.