MonitorManager: add support for persistent monitor configurations

Add a new object, MetaMonitorConfig, that takes care of converting
between the logical configurations stored in monitors.xml and
the HW resources exposed by MonitorManager.
This commit includes loading and saving of configurations, but
still missing is the actual CRTC assignments and a default
configuration when none is found in the file.

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