shew: Add small library for dealing with external windows

In order to support OpenExtensionPrefs()'s parentWindow parameter,
we will need the ability to make a window transient to an external
window identified by a string handle.

This takes the corresponding code from xdg-desktop-portal-gtk and
brings it into an introspectable form, likewise the counterpart in
libportal to export a string handle for a GtkWindow.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1087

subprojects/shew/src/shew-external-window-wayland.h

@@ -0,0 +1,30 @@
+/*
+ * Copyright © 2016 Red Hat, Inc
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authors:
+ *       Jonas Ådahl <jadahl@redhat.com>
+ */
+
+#pragma once
+
+#include <glib-object.h>
+
+#include "shew-external-window.h"
+
+#define SHEW_TYPE_EXTERNAL_WINDOW_WAYLAND (shew_external_window_wayland_get_type ())
+G_DECLARE_FINAL_TYPE (ShewExternalWindowWayland, shew_external_window_wayland, SHEW, EXTERNAL_WINDOW_WAYLAND, ShewExternalWindow)
+
+ShewExternalWindowWayland *shew_external_window_wayland_new (const char *handle_str);