Add a ReturnToGroundLayerOnIdle flag to CustomMovementLayers

Co-authored-by: Mazar Farran <farranmazar@gmail.com>
Co-authored-by: Paul Chote <paul@chote.net>

OpenRA.Mods.Common/Traits/Mobile.cs

@@ -672,7 +672,13 @@ namespace OpenRA.Mods.Common.Traits
 
 		void INotifyBecomingIdle.OnBecomingIdle(Actor self)
 		{
-			if (TopLeft.Layer == 0)
+			if (self.Location.Layer == 0)
+				return;
+
+			var cml = self.World.WorldActor.TraitsImplementing<ICustomMovementLayer>()
+				.First(l => l.Index == self.Location.Layer);
+
+			if (!cml.ReturnToGroundLayerOnIdle)
 				return;
 
 			var moveTo = ClosestGroundCell();