Introduce MetaContextMain

This object intends to replace the scattered functions that are used to
make up what is effectively a "mutter context". It takes care of the
command line arguments that is now done in main.c, persistant virtual
monitors, and the like.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>

src/core/meta-context-private.h

@@ -23,7 +23,6 @@
 
 #include "meta/meta-backend.h"
 #include "meta/meta-context.h"
-#include "meta/meta-enums.h"
 
 struct _MetaContextClass
 {