Move VisibilityType enum to TraitsInterfaces

VisibilityTypes are used by several other traits as well, so it makes more sense to move it to a common, shared location.

OpenRA.Mods.Common/TraitsInterfaces.cs

@@ -19,6 +19,8 @@ using OpenRA.Traits;
 
 namespace OpenRA.Mods.Common.Traits
 {
+	public enum VisibilityType { Footprint, CenterPosition, GroundPosition }
+
 	public enum AttackDelayType { Preparation, Attack }
 
 	public interface IQuantizeBodyOrientationInfo : ITraitInfo