Made all traitInfo fields readonly

This came up while working on the new documentation generation and comparing the results to ORAIDE's own code parser.

OpenRA.Mods.Common/Traits/Sellable.cs

@@ -28,7 +28,7 @@ namespace OpenRA.Mods.Common.Traits
 
 		[NotificationReference("Speech")]
 		[Desc("The audio notification type to play.")]
-		public string Notification = null;
+		public readonly string Notification = null;
 
 		[Desc("Whether to show the cash tick indicators rising from the actor.")]
 		public readonly bool ShowTicks = true;