This adds a way to define a way, at the system level, to define a policy
of how monitor configuration files are loaded.
The intended use case is to e.g. either prefer system level monitor
configurations before user levels, or only allow system level
configurations.
Examples:
Prefer system over user level configurations:
<monitors version="2">
<policy>
<stores>
<store>system</store>
<store>user</store>
</stores>
</policy>
<configuration>
...
</configuration>
</monitors>
Only allow system level configurations:
<monitors version="2">
<policy>
<stores>
<store>system</store>
</stores>
</policy>
<configuration>
...
</configuration>
</monitors>
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2030>
Check that configurations where monitors are disabled are properly
used. Also test that old configurations with explicitly disabled
outputs are migrated properly.
https://bugzilla.gnome.org/show_bug.cgi?id=787629
So far some basic testing, including:
* Test that the migrated configuration is applicable
* Test that a monitors.xml with multiple configurations are translated
* Test rotation
* Test tiled monitor discovery (well, test a made up tiled monitor
configuration since I don't have a real one)
https://bugzilla.gnome.org/show_bug.cgi?id=777732
