window: set sensitivity based on display capability
    
    A following patch is adding a new display (VTE) that won't have the
    send_key() or screenshot() callbacks. Activating those menu/actions
    would lead to nothing or a crash. I chose to keep the UI consistent
    for all display, but disable the menu sensitivity.
    
    Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Acked-by: Victor Toso <victortoso@redhat.com>