Reverse handling of XRandR events between Screen and MonitorManager

Now MonitorManager does its own handling of XRandR events, which
means we no longer handle ConfigureNotify on the root window.
MetaScreen reacts to MonitorManager::monitor-changed and updates
its internal state, including the new size.

This paves the way for doing display configuration using only
the dummy backend, which would allow testing wl_output interfaces.

https://bugzilla.gnome.org/show_bug.cgi?id=705670