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>
Monitor configuration check tests can be very complex and in case of
failures we can't easily catch where a failure happened without entering
in debug mode, something that isn't always an option in CI or external
builders.
So add more debug statements in configuration check functions and use
macros to ensure that we print the caller function and location on more
complex check functions.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/522>
This will require some symbol exporting, but the benefit is that have
better control of what external test cases can do in terms of creating
more testing specific contexts.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>
