ui-frame: Add support for style variants

Rather than sharing a single style context between all frames, use
a default style and one style per encountered variant (as determined
by the _GTK_THEME_VARIANT property), so that colors from the GTK+ style
are picked from the correct theme variant.

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

src/ui/frames.h

@@ -98,6 +98,9 @@ struct _MetaFrames
   guint tooltip_timeout;
   MetaUIFrame *last_motion_frame;
 
+  GtkStyleContext *normal_style;
+  GHashTable *style_variants;
+
   int expose_delay_count;
 
   int invalidate_cache_timeout_id;