Expose targetline colors to yaml.

All targetlines can now be set to a custom color in yaml or set to be invisible.
All automated behaviours including scripted activities now have no visible target lines.

OpenRA.Mods.Common/Activities/Air/ReturnToBase.cs

@@ -131,7 +131,7 @@ namespace OpenRA.Mods.Common.Activities
 		public override IEnumerable<TargetLineNode> TargetLineNodes(Actor self)
 		{
 			if (ChildActivity == null)
-				yield return new TargetLineNode(Target.FromActor(dest), Color.Green);
+				yield return new TargetLineNode(Target.FromActor(dest), aircraft.Info.TargetLineColor);
 			else
 				foreach (var n in ChildActivity.TargetLineNodes(self))
 					yield return n;