Dispose SequenceSet when we're done with it.

Utility rules that do something on a map and exit
are left without explicit disposing, as they will
be cleaned up immediately anyway.

OpenRA.Mods.Common/Widgets/Logic/Editor/NewMapLogic.cs

@@ -70,6 +70,7 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 
 				Action<string> afterSave = uid =>
 				{
+					map.Dispose();
 					Game.LoadEditor(uid);
 
 					Ui.CloseWindow();