window: Add a MetaWindowConfig type

This is intended to be used in place of the window rect and fullscreen
flags.

That will also allow for a pre-configuration signal to be added, passing
the configuration so that a plugin can tweak the configuration before it
gets applied first.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4076>

src/core/meta-window-config-private.h

@@ -0,0 +1,23 @@
+/*
+ * Copyright (C) 2024 Red Hat Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#pragma once
+
+#include "meta/meta-window-config.h"
+
+MetaWindowConfig * meta_window_config_initial_new (void);