lib: add struct paddings to be able to maintain ABI compatibility

Aim for 8 pointers on each struct, wich exceptions:
- 12 pointers for already crowded/important ones
- 4 pointers for empty ones that are used for proxies; we are
  unlikely to add any virtual methods on these classes ever

lib/wp/component-loader.h

@@ -30,6 +30,9 @@ struct _WpComponentLoaderClass
 
   gboolean (*load) (WpComponentLoader * self, const gchar * component,
       const gchar * type, GVariant * args, GError ** error);
+
+  /*< private >*/
+  WP_PADDING(6)
 };
 
 G_END_DECLS