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/Crates/ExplodeCrateAction.cs

@@ -19,7 +19,7 @@ namespace OpenRA.Mods.Common.Traits
 		[WeaponReference]
 		[FieldLoader.Require]
 		[Desc("The weapon to fire upon collection.")]
-		public string Weapon = null;
+		public readonly string Weapon = null;
 
 		public override object Create(ActorInitializer init) { return new ExplodeCrateAction(init.Self, this); }
 	}