Move AirThreshold to Warhead

To reduce duplication and for later use in more warheads.

OpenRA.Mods.Common/Warheads/Warhead.cs

@@ -46,6 +46,9 @@ namespace OpenRA.Mods.Common.Warheads
 		[Desc("Can this warhead affect the actor that fired it.")]
 		public readonly bool AffectsParent = false;
 
+		[Desc("If impact is above this altitude, warheads that would affect terrain ignore terrain target types (and either do nothing or perform their own checks).")]
+		public readonly WDist AirThreshold = new WDist(128);
+
 		[Desc("Delay in ticks before applying the warhead effect.", "0 = instant (old model).")]
 		public readonly int Delay = 0;