Add an ExitDelay field to allow doors to open before exiting.

OpenRA.Mods.Common/Traits/Buildings/Exit.cs

@@ -26,6 +26,9 @@ namespace OpenRA.Mods.Common.Traits
 
 		[Desc("AttackMove to a RallyPoint or stay where you are spawned.")]
 		public readonly bool MoveIntoWorld = true;
+
+		[Desc("Number of ticks to wait before moving into the world.")]
+		public readonly int ExitDelay = 0;
 	}
 
 	public class Exit { }

OpenRA.Mods.Common/Traits/Production.cs

@@ -77,6 +77,9 @@ namespace OpenRA.Mods.Common.Traits
 				{
 					if (exitinfo.MoveIntoWorld)
 					{
+						if (exitinfo.ExitDelay > 0)
+							newUnit.QueueActivity(new Wait(exitinfo.ExitDelay));
+
 						newUnit.QueueActivity(move.MoveIntoWorld(newUnit, exit));
 						newUnit.QueueActivity(new AttackMoveActivity(
 							newUnit, move.MoveTo(exitLocation, 1)));