Bug fix: system can't be re-added to list after removeAll().

If a system's parent is already the current list, `add()` will skip adding that system. Normally this is fine because the parent is cleared if the system is removed, but `removeAll()` skipped that step, so you could have a system incorrectly claim to be in a list.

src/echoes/SystemList.hx

@@ -191,6 +191,7 @@ class SystemList extends System {
 	public function removeAll():SystemList {
 		for(system in systems) {
 			system.__deactivate__();
+			system.parent = null;
 		}
 		systems.resize(0);