MetaMonitorConfigStore provides an XML storage mechanism for
MetaMonitorConfigManager. It stores configuration files defined in the
same level as the MetaMonitorsConfig format, i.e. refers to high level
"monitors" and "monitor modes" instead of connectors and CRTCs.
Only reading custom files are implemented and so far unused.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
