-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

import/ecs/system.di

@@ -0,0 +1,26 @@
+// D import file generated from 'source\ecs\system.d'
+module ecs.system;
+import ecs.entity;
+import ecs.manager;
+struct System
+{
+	export bool enabled();
+	export void enable();
+	export void disable();
+	export int priority();
+	package 
+	{
+		bool m_enabled = false;
+		int m_priority;
+		void* m_system_pointer;
+		ushort[] m_components;
+		ushort[] m_optional_components;
+		void* m_update;
+		void function(void* system_pointer) m_enable;
+		void function(void* system_pointer) m_disable;
+		void function(void* system_pointer) m_create;
+		void function(void* system_pointer) m_destroy;
+		void function(void* system_pointer) m_begin;
+		void function(void* system_pointer) m_end;
+	}
+}