ui: Add meta_ui_update_frame_style()

This method allows forcing a style update of a particular frame
from the core, so that it can pick up style variants.

https://bugzilla.gnome.org/show_bug.cgi?id=645355

src/ui/ui.c

@@ -444,6 +444,13 @@ meta_ui_unflicker_frame_bg (MetaUI *ui,
                             target_width, target_height);
 }
 
+void
+meta_ui_update_frame_style (MetaUI  *ui,
+                            Window   xwindow)
+{
+  meta_frames_update_frame_style (ui->frames, xwindow);
+}
+
 void
 meta_ui_repaint_frame (MetaUI *ui,
                        Window xwindow)