MetaFrameBorders: Add meta_frame_borders_clear

Just a quick little commit to help clean things up for when we add invisible
borders. Additionally, do a little housekeeping in preview-widget as well.

https://bugzilla.gnome.org/show_bug.cgi?id=644930

src/ui/ui.c

@@ -736,10 +736,7 @@ meta_ui_theme_get_frame_borders (MetaUI *ui,
     }
   else
     {
-      borders->visible.top = 0;
-      borders->visible.bottom = 0;
-      borders->visible.left = 0;
-      borders->visible.right = 0;
+      meta_frame_borders_clear (borders);
     }
 
   if (style != NULL)