-onBegin() return type was changed from void to bool

-disabling system execution in onBegin() funciton

source/ecs/system.d

@@ -47,6 +47,9 @@ struct System
 
 package:
 
+    ///should system be executed in current update?
+    bool execute = true;
+
     ///should system update and catch events?
     bool m_enabled = false;
     ///system priority