context: Add setup phase

During this phase, the backend is created and configured. Currently only
configured, but will gain more logic that currently main.c does with
various helpers.

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

src/core/meta-context-private.h

@@ -36,6 +36,9 @@ struct _MetaContextClass
 
   MetaCompositorType (* get_compositor_type) (MetaContext *context);
 
+  gboolean (* setup) (MetaContext  *context,
+                      GError      **error);
+
   MetaBackend * (* create_backend) (MetaContext  *context,
                                     GError      **error);
 };