window: Only advertise fullscreen menu item if client supports it

window: Only advertise fullscreen menu item if client supports it

If the client doesn't set a fullscreen handler, we can't go fullscreen
and shouldn't advertise that in the window menu.  The menu implementation is
a little simplistic, so we just move the "Fullscreen" entry to the end of
the list and don't count it if we don't want it in the menu.

https://bugs.freedesktop.org/show_bug.cgi?id=47751
https://bugs.freedesktop.org/show_bug.cgi?id=48106