[StShadow] Add reference counting

If a shadow property is inherited from a parent, multiple StThemeNodes
share a common StShadow. It would be possible to use st_shadow_copy()
for this purpose, but proper reference counting is nicer.

https://bugzilla.gnome.org/show_bug.cgi?id=624384

src/st/st-theme-node.c

@@ -99,7 +99,7 @@ st_theme_node_finalize (GObject *object)
 
   if (node->shadow)
     {
-      st_shadow_free (node->shadow);
+      st_shadow_unref (node->shadow);
       node->shadow = NULL;
     }