monitor-config: Keep track of disabled monitors for stored configs

When saving and restoring monitor configurations, we must take disabled
monitors into account, as otherwise one cannot store/restore a
configuration where one or more monitors are explicitly disabled. Make
this possible by adding a <disabled> element to the <configure> element
which lists the monitors that are explicitly disabled. These ones are
included when generating the configuration key, meaning they'll be
picked up correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=787629