Add plumbing for notifying traits of movement

More precisely, about start and stop of movement.

OpenRA.Mods.Common/Activities/Move/VisualMoveIntoTarget.cs

@@ -33,7 +33,6 @@ namespace OpenRA.Mods.Common.Activities
 		protected override void OnFirstRun(Actor self)
 		{
 			targetStartPos = target.Positions.PositionClosestTo(self.CenterPosition);
-			mobile.IsMoving = true;
 		}
 
 		public override Activity Tick(Actor self)
@@ -84,11 +83,6 @@ namespace OpenRA.Mods.Common.Activities
 			return this;
 		}
 
-		protected override void OnLastRun(Actor self)
-		{
-			mobile.IsMoving = false;
-		}
-
 		public override IEnumerable<Target> GetTargets(Actor self)
 		{
 			yield return target;