Add support to configure the logical monitor scale. With this, it
becomes possible to override the automatically calculated scaling
number per logical monitor.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
The default (calculated) scale is derived from the output, but
ultimately set via the monitor scale. This will enable config files to
override the scale. Yet to be done is handling when a scale is not
supported by a backend (i.e. the X11 backend).
https://bugzilla.gnome.org/show_bug.cgi?id=777732
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
