-added filterEntity callback (used to filter EntityInfos for system, better control than simply @optional)

-removed some redundant code (two times same code)
-added some common functions

source/bubel/ecs/system.d

@@ -194,6 +194,8 @@ package:
     void* m_remove_entity;
     void* m_change_entity;
 
+    void* m_filter_entity;
+
     //void function(ref EntityManager.CallData data) m_initialize;
     //void function(ref EntityManager.CallData data) m_deinitilize;
     void* m_initialize;