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.Common/Traits/SupportPowers/SupportPower.cs

@@ -32,16 +32,33 @@ namespace OpenRA.Mods.Common.Traits
 		public readonly string[] Prerequisites = { };
 
 		public readonly string BeginChargeSound = null;
+
+		[NotificationReference("Speech")]
 		public readonly string BeginChargeSpeechNotification = null;
+
 		public readonly string EndChargeSound = null;
+
+		[NotificationReference("Speech")]
 		public readonly string EndChargeSpeechNotification = null;
+
 		public readonly string SelectTargetSound = null;
+
+		[NotificationReference("Speech")]
 		public readonly string SelectTargetSpeechNotification = null;
+
 		public readonly string InsufficientPowerSound = null;
+
+		[NotificationReference("Speech")]
 		public readonly string InsufficientPowerSpeechNotification = null;
+
 		public readonly string LaunchSound = null;
+
+		[NotificationReference("Speech")]
 		public readonly string LaunchSpeechNotification = null;
+
 		public readonly string IncomingSound = null;
+
+		[NotificationReference("Speech")]
 		public readonly string IncomingSpeechNotification = null;
 
 		[Desc("Defines to which players the timer is shown.")]