Introduce new monitor configuration system

The new monitor configuration system (MetaMonitorConfigManager) aims to
replace the current MetaMonitorConfig. The main difference between the
two is that MetaMonitorConfigManager works with higher level input
(MetaMonitor, MetaMonitorMode) instead of directly looking at the CRTC
and connector state. It still produces CRTC and connector configuration
later applied by the respective backends.

Other difference the new system aims to introduce is that the
configuration system doesn't manipulate the monitor manager state; that
responsibility is left for the monitor manager to handle (it only
manages configuration and creates CRTC/connector assignments, it
doesn't apply anything).

The new configuration system allows backends to not rely on deriving the
current configuration from the CRTC/connector state, as this may no longer be
possible (i.e. when using KMS and multiple framebuffers).

The MetaMonitorConfigManager system is so far disabled by default, as
it does not yet have all the features of the old system, but eventually
it will replace MetaMonitorConfig which will at that point be removed.
This will make it possible to remove old hacks introduced due to
limitations in the old system.

https://bugzilla.gnome.org/show_bug.cgi?id=777732