add common version of AreMutualAllies to WorldUtils

OpenRA.Game/WorldUtils.cs

@@ -158,5 +158,11 @@ namespace OpenRA
 					Log.Write("perf", text, x, dt * 1000, Game.LocalTick);
 			});
 		}
+
+		public static bool AreMutualAllies( Player a, Player b )
+		{
+			return a.Stances[b] == Stance.Ally &&
+				b.Stances[a] == Stance.Ally;
+		}
 	}
 }