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/DebugPauseState.cs

@@ -18,7 +18,7 @@ namespace OpenRA.Traits
 
 	public class DebugPauseState : ISync
 	{
-		World world;
+		readonly World world;
 		[Sync] public bool Paused { get { return world.Paused; } }
 		public DebugPauseState(World world) { this.world = world; }
 	}