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/meta/meta-context.h

@@ -35,4 +35,8 @@ gboolean meta_context_configure (MetaContext   *context,
                                  char        ***argv,
                                  GError       **error);
 
+META_EXPORT
+gboolean meta_context_setup (MetaContext  *context,
+                             GError      **error);
+
 #endif /* META_CONTEXT_H */