Add a lint test for audio notifications.

Only traits are linted - the UI still hardcodes
too many audio references for this to be worthwhile.

OpenRA.Mods.D2k/Traits/AttackSwallow.cs

@@ -32,6 +32,7 @@ namespace OpenRA.Mods.D2k.Traits
 
 		public readonly string WormAttackSound = "WORM.WAV";
 
+		[NotificationReference("Speech")]
 		public readonly string WormAttackNotification = "WormAttack";
 
 		public override object Create(ActorInitializer init) { return new AttackSwallow(init.Self, this); }