-registering same system multiple times only replace pointer for callback

-added export attributes (required by windows to make DLL library, not work at now due to DMD bugs)
-interface D files to import

source/ecs/entity.d

@@ -26,7 +26,7 @@ struct Entity
     }
 }
 
-struct EntityTemplate
+export struct EntityTemplate
 {
     ubyte[] entity_data;
     EntityManager.EntityInfo* info;