filechooser: Attach GSettings object to GtkSettings

... instead of keeping one per GtkFileChooserDefault. This allows using
it in other places, too.

gtk/gtkfilechooserprivate.h

@@ -280,8 +280,6 @@ struct _GtkFileChooserDefault
   gint sort_column;
   GtkSortType sort_order;
 
-  GSettings *settings;
-
 #if 0
   GdkDragContext *shortcuts_drag_context;
   GSource *shortcuts_drag_outside_idle;