app: add "machine" UI
    
    Add a new "Machine" menu, which allows to Pause/Reset/Power Down a VM.
    
    The menu is only visible if "vm-ui" app property is set.
    
    When the application quits, it will also send a quit action to the VM.
    
    This is a similar behaviour/UI as qemu -display gtk.
    
    Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Acked-by: Victor Toso <victortoso@redhat.com>