Port extensions app and portal to GTK4

With the previous preparations in place, it is time to take the plunge.

As both the app and the portal use the same small library for handling
external windows, port everything at once to avoid the hassle of building
and installing two versions of the library.

With the portal using GTK4 now, all extensions must port their preference
widgets as well.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1495>

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

@@ -21,7 +21,7 @@
 #include <gdk/gdk.h>
 
 #ifdef GDK_WINDOWING_WAYLAND
-#include <gdk/gdkwayland.h>
+#include <gdk/wayland/gdkwayland.h>
 #endif
 
 #include "shew-external-window-wayland.h"
@@ -77,14 +77,14 @@ shew_external_window_wayland_new (const char *handle_str)
 
 static void
 shew_external_window_wayland_set_parent_of (ShewExternalWindow *external_window,
-                                            GdkWindow      *child_window)
+                                            GdkSurface         *child_surface)
 {
   ShewExternalWindowWayland *external_window_wayland =
     SHEW_EXTERNAL_WINDOW_WAYLAND (external_window);
   char *handle_str = external_window_wayland->handle_str;
 
 #ifdef GDK_WINDOWING_WAYLAND
-  if (!gdk_wayland_window_set_transient_for_exported (child_window, handle_str))
+  if (!gdk_wayland_toplevel_set_transient_for_exported (GDK_WAYLAND_TOPLEVEL (child_surface), handle_str))
     g_warning ("Failed to set portal window transient for external parent");
 #endif
 }