popupMenu: Simplify allocation code

Use ClutterActor.allocate_align_fill() so we don't have to do
this math ourselves. At the same time, clean up the RTL handling
so that it's easier to follow.

https://bugzilla.gnome.org/show_bug.cgi?id=702539

src/st/st-widget.h

@@ -151,7 +151,6 @@ StThemeNode *         st_widget_peek_theme_node           (StWidget        *widg
 GList *               st_widget_get_focus_chain           (StWidget        *widget);
 void                  st_widget_paint_background          (StWidget        *widget);
 
-
 /* debug methods */
 char  *st_describe_actor       (ClutterActor *actor);
 void   st_set_slow_down_factor (gfloat factor);
@@ -169,6 +168,12 @@ void                  st_widget_set_accessible_name      (StWidget    *widget,
                                                           const gchar *name);
 const gchar *         st_widget_get_accessible_name      (StWidget    *widget);
 
+/* utility methods */
+void st_get_align_factors (StAlign   x_align,
+                           StAlign   y_align,
+                           gdouble  *x_align_out,
+                           gdouble  *y_align_out);
+
 G_END_DECLS
 
 #endif /* __ST_WIDGET_H__ */