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/frames.c

@@ -1126,6 +1126,20 @@ meta_frames_set_title (MetaFrames *frames,
   invalidate_whole_window (frames, frame);
 }
 
+void
+meta_frames_update_frame_style (MetaFrames *frames,
+                                Window      xwindow)
+{
+  MetaUIFrame *frame;
+
+  frame = meta_frames_lookup_window (frames, xwindow);
+
+  g_assert (frame);
+
+  meta_frames_attach_style (frames, frame);
+  invalidate_whole_window (frames, frame);
+}
+
 void
 meta_frames_repaint_frame (MetaFrames *frames,
                            Window      xwindow)