Attaching global menus

Some Linux desktops (but not all) support a concept of a global menu where the applications main menu is not drawn in the window but at the top of the screen (OS X style).

KDE had their own implementation in the QPlatformTheme, but mid-Qt5 an implementation entered QGenericUnixServices for some desktops. Probably for Unity at the time.

KDE had extended the DBus menubar protocol to report the DBus service and path on the window itself, this makes looking it up easier than going through a broker.

This is necessary on wayland as we don't have a meaningful concept of a "window ID" to send to the desktop over the DBus interface. The implementation in QGenericUnixServices isn't directly usable on wayland on any platform.