-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/events.di

@@ -0,0 +1,9 @@
+// D import file generated from 'source\ecs\events.d'
+module ecs.events;
+struct Event
+{
+	uint type;
+}
+class EventManager
+{
+}