Fix GTK4 deprecations in flash.c

src/flash.c

@@ -83,9 +83,7 @@ mp_flash_gtk_init(GDBusConnection *conn)
         gtk_window_set_decorated(GTK_WINDOW(window), FALSE);
         gtk_window_fullscreen(GTK_WINDOW(window));
 
-        GtkStyleContext *context;
-        context = gtk_widget_get_style_context(window);
-        gtk_style_context_add_class(context, "flash");
+        gtk_widget_add_css_class(window, "flash");
 
         flash_window = window;
 }
@@ -162,7 +160,7 @@ show_display_flash(MPFlash *flash)
         if (!flash_window)
                 return false;
 
-        gtk_widget_show(flash_window);
+        gtk_widget_set_visible(flash_window, true);
 
         // First get brightness and then set brightness to 100%
         if (!dbus_brightness_proxy)
@@ -204,7 +202,7 @@ hide_display_flash(MPFlash *flash)
         if (!flash_window)
                 return false;
 
-        gtk_widget_hide(flash_window);
+        gtk_widget_set_visible(flash_window, false);
         set_display_brightness(dbus_old_brightness);
 
         return false;