Rewrite Enter and related activities.

The unit will now path to the nearest adjacent cell, drag inside, do the inner activity, then (if still alive) drag back to its original
location.

This fixes:
 - Mismatch in logic between Enter and MoveAdjacentTo,
   which causes an infinite loop.
 - Building capturing failing from certain directions.
 - Being unable to enter buildings on unpathable tiles.
 - Units being stranded inside a building if the requirements
   for the inner order aren't met.

OpenRA.Mods.RA/Infiltrates.cs

@@ -67,8 +67,7 @@ namespace OpenRA.Mods.RA
 				self.SetTargetLine(Target.FromOrder(order), Color.Red);
 				
 				self.CancelActivity();
-				self.QueueActivity(new Enter(order.TargetActor));
-				self.QueueActivity(new Infiltrate(order.TargetActor));
+				self.QueueActivity(new Enter(order.TargetActor, new Infiltrate(order.TargetActor)));
 			}
 		}