Revert "monitor-manager-xrandr: Don't do extra work on RRScreenChangeNotify"

This reverts commit 47e339b46e80f01c2a4d020dbef7341e5a02b7eb. The
approach that was used to reduce the amount of work we do on RR events
to the necessary minimum is flawed. It assumes that, when the first
event we see where the retrieved XRRScreenResources.timestamp is
bigger than the previous, we already have all the data we need to
rebuild our view of the world.

That isn't true however, because the X server sends
RRScreenChangeNotify events for every step of the configuration
change, i.e. it lacks an atomic reconfiguration API. In particular, if
the X screen size is one of the changes, when we rebuild our state and
emit monitors-changed, the X screen size might still be the previous
one and since we stop updating ourselves until another reconfiguration
happens (noticed by looking at XRRScreenResources.timestamp) we end up
with the wrong idea of the X screen size.

https://bugzilla.gnome.org/show_bug.cgi?id=738630