Instead of using a environment variable, add a new 'experimental
feature' gsetting keyword "monitor-config-manager" that enables the use
of the new MetaMonitorConfigManager. This commit also makes it possible
to switch between the two systems without restarting mutter.
The D-Bus API is disabled when the experimental feature is not enabled,
and clients trying to access it will get a access-denied error in
response. A new property 'IsExperimentalApiEnabled' is added to let the
D-Bus client know whether it is possible to use the experimental API or
not.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
Both the monitor unit tests and monitor store unit tests will want to
check whether the config manager is used and set custom configuration
files.
https://bugzilla.gnome.org/show_bug.cgi?id=777732