frames/window-tracker: Reinstate dark titlebar support

Merge request !2541 [1] introduced support for integrating Mutter
frames with the dark style. This was lost after moving frames into
a separate client.

Bring that back. Don't depend on gdesktop-enums as it brings GTK3
into the header chain.

[1] https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2541

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2739>

src/frames/meson.build

@@ -9,6 +9,7 @@ x11_frames_sources = [
 x11_frames = executable('mutter-x11-frames',
   sources: x11_frames_sources,
   dependencies: [
+    gsettings_desktop_schemas_dep,
     gtk4_dep,
     x11_dep,
     xext_dep,

src/frames/meta-window-tracker.c

@@ -23,6 +23,7 @@
 
 #include "meta-frame.h"
 
+#include <gdesktop-enums.h>
 #include <gdk/x11/gdkx.h>
 #include <X11/Xatom.h>
 #include <X11/extensions/XInput2.h>
@@ -30,6 +31,9 @@
 struct _MetaWindowTracker
 {
   GObject parent_instance;
+
+  GSettings *interface_settings;
+
   GdkDisplay *display;
   GHashTable *frames;
   GHashTable *client_windows;
@@ -84,6 +88,23 @@ meta_window_tracker_get_property (GObject    *object,
     }
 }
 
+static void
+on_color_scheme_changed_cb (GSettings         *interface_settings,
+                            GParamSpec        *pspec,
+                            MetaWindowTracker *window_tracker)
+{
+  GDesktopColorScheme color_scheme;
+  gboolean is_dark;
+
+  color_scheme = g_settings_get_enum (window_tracker->interface_settings,
+                                     "color-scheme");
+  is_dark = color_scheme == G_DESKTOP_COLOR_SCHEME_PREFER_DARK;
+
+  g_object_set (gtk_settings_get_default (),
+                "gtk-application-prefer-dark-theme", is_dark,
+                NULL);
+}
+
 static void
 set_up_frame (MetaWindowTracker *window_tracker,
               Window             xwindow)
@@ -346,6 +367,7 @@ meta_window_tracker_finalize (GObject *object)
 {
   MetaWindowTracker *window_tracker = META_WINDOW_TRACKER (object);
 
+  g_clear_object (&window_tracker->interface_settings);
   g_clear_pointer (&window_tracker->frames,
                    g_hash_table_unref);
   g_clear_pointer (&window_tracker->client_windows,
@@ -382,6 +404,12 @@ meta_window_tracker_class_init (MetaWindowTrackerClass *klass)
 static void
 meta_window_tracker_init (MetaWindowTracker *window_tracker)
 {
+  window_tracker->interface_settings = g_settings_new ("org.gnome.desktop.interface");
+  g_signal_connect (window_tracker->interface_settings,
+                    "changed::color-scheme",
+                    G_CALLBACK (on_color_scheme_changed_cb),
+                    window_tracker);
+
   window_tracker->frames =
     g_hash_table_new_full (NULL, NULL, NULL,
                            (GDestroyNotify) gtk_window_destroy);