Remove st_widget accessors for background and border actors

StButton animated the background for button transitions; since these aren't
presently part of the shell design, simply remove them.  We can readd
these in the future.

StTooltip should probably have :vertical and :horizontal pseudo classes
to make the arrow work but it should still function.

https://bugzilla.gnome.org/show_bug.cgi?id=607500

src/st/st-button.h

@@ -68,8 +68,7 @@ struct _StButtonClass
   /* vfuncs, not signals */
   void (* pressed)    (StButton     *button);
   void (* released)   (StButton     *button);
-  void (* transition) (StButton     *button,
-                       ClutterActor *old_bg);
+  void (* transition) (StButton     *button);
 
   /* signals */
   void (* clicked) (StButton *button);