Add st_widget_peek_theme_node()

Sometimes it's useful to get the theme node if there is one and do
nothing and wait for the ::style-changed signal if there is no theme
node. Add st_widget_peek_theme_node() that just gets the current
theme node if available. The caller must handle a %NULL return.

https://bugzilla.gnome.org/show_bug.cgi?id=633865

src/st/st-widget.h

@@ -151,6 +151,7 @@ gboolean              st_widget_navigate_focus            (StWidget        *widg
 /* Only to be used by sub-classes of StWidget */
 void                  st_widget_style_changed             (StWidget        *widget);
 StThemeNode *         st_widget_get_theme_node            (StWidget        *widget);
+StThemeNode *         st_widget_peek_theme_node           (StWidget        *widget);
 
 /* debug methods */
 char  *st_describe_actor       (ClutterActor *actor);