General uncontroversial cleanup:

- Made private methods static where possible (runtime can elide checking the object for null).
- Declared attribute classes as sealed (allows reflection on attributes to complete faster).
- Moved some static cctor's into field initializers (static cctor's are slower than static field initializers).
- Made classes static if they contained only static methods (can't create instances of useless objects).
- Use inferable Exts.Lazy and not new Lazy<T>().
- Added required STAThread attribute to CrashDialog.
- Removed unused parameters in private methods.
- Added Serializable attribute to exceptions.
- Added parameter name in calls to ArgumentNullException.
- Use of as operator instead of is + cast.
- Changed (x as Foo).Bar anti-pattern into ((Foo)x).Bar. Results in sensible cast exceptions on error rather than null dereferences.
- Removed unused method in NullShader.

OpenRA.Mods.RA/ExternalCaptures.cs

@@ -56,7 +56,7 @@ namespace OpenRA.Mods.RA
 			return new Order(order.OrderID, self, queued) { TargetActor = target.Actor };
 		}
 
-		bool IsValidOrder(Actor self, Order order)
+		static bool IsValidOrder(Actor self, Order order)
 		{
 			// Not targeting an actor
 			if (order.ExtraData == 0 && order.TargetActor == null)