Simplify return statements.

Remove redundant ‘this’.
Remove unused using directives.
Simplify LINQ chains.
Add some trait property descriptions.
Add readonly where viable.
Add fullstops to some yaml descriptions.

OpenRA.Game/Traits/LintAttributes.cs

@@ -12,7 +12,7 @@ using System;
 
 namespace OpenRA.Traits
 {
-	/* attributes used by RALint to understand the rules */
+	/* attributes used by OpenRA.Lint to understand the rules */
 
 	[AttributeUsage(AttributeTargets.Field)]
 	public sealed class ActorReferenceAttribute : Attribute { }