[St] add st_describe_actor, for debugging

This can be used when adding debug printfs, to get clear descriptions of
actors.

Also update the Looking Glass inspector to use it

https://bugzilla.gnome.org/show_bug.cgi?id=621668

src/st/st-widget.h

@@ -141,6 +141,8 @@ void                  st_widget_set_direction             (StWidget        *self
 void                  st_widget_style_changed             (StWidget        *widget);
 StThemeNode *         st_widget_get_theme_node            (StWidget        *widget);
 
+/* debug method */
+char *st_describe_actor (ClutterActor *actor);
 
 G_END_DECLS