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/object.h

@@ -74,6 +74,9 @@ struct _WpObjectClass
       WpObjectFeatures missing);
 
   void (*deactivate) (WpObject * self, WpObjectFeatures features);
+
+  /*< private >*/
+  WP_PADDING(8)
 };
 
 WP_API