Don't emit 'monitor-geometry-changed' for disabled displays
    
    When a display widget receives a new size allocation, we generally emit
    a monitor-geometry-changed signal so that we can reconfigure the
    displays. But if the widget for a *disabled* display is allocated,
    there's no reason to send down a new configuration. We don't need to
    emit this signal. This doesn't currently cause any problems, but I ran
    into issues while testing some other uncommitted changes.