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/SelectionDecorations.cs

@@ -28,7 +28,7 @@ namespace OpenRA.Traits
 		static readonly string[] TagStrings = { "", "tag-fake", "tag-primary" };
 
 		public SelectionDecorationsInfo Info;
-		Actor self;
+		readonly Actor self;
 
 		public SelectionDecorations(Actor self, SelectionDecorationsInfo info)
 		{