-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/package.di

@@ -0,0 +1,7 @@
+// D import file generated from 'source\ecs\package.d'
+module ecs;
+public import ecs.manager;
+public import ecs.entity;
+public import ecs.system;
+import ecs.id_manager;
+import ecs.events;