Squash epic memory leak

StThemeNode holds a reference to its parent, but we never released
that reference.  This could cause us to hold onto whole chains
of theme nodes with rather dire memory usage implications.

Also move the other g_object_unref into _dispose.

https://bugzilla.gnome.org/show_bug.cgi?id=614660

src/st/st-widget.c

@@ -253,6 +253,7 @@ st_widget_dispose (GObject *gobject)
 
   if (priv->theme_node)
     {
+      g_object_run_dispose (G_OBJECT (priv->theme_node));
       g_object_unref (priv->theme_node);
       priv->theme_node = NULL;
     }