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.Game/Widgets/WidgetLoader.cs

@@ -71,7 +71,7 @@ namespace OpenRA
 			return widget;
 		}
 
-		Widget NewWidget(string widgetType, WidgetArgs args)
+		static Widget NewWidget(string widgetType, WidgetArgs args)
 		{
 			widgetType = widgetType.Split('@')[0];
 			return Game.modData.ObjectCreator.CreateObject<Widget>(widgetType + "Widget", args);

OpenRA.Game/Widgets/WorldInteractionControllerWidget.cs

@@ -229,7 +229,7 @@ namespace OpenRA.Widgets
 			return false;
 		}
 
-		IEnumerable<Actor> SelectActorsInBox(World world, int2 a, int2 b, Func<Actor, bool> cond)
+		static IEnumerable<Actor> SelectActorsInBox(World world, int2 a, int2 b, Func<Actor, bool> cond)
 		{
 			return world.ScreenMap.ActorsInBox(a, b)
 				.Where(x => x.HasTrait<Selectable>() && x.Trait<Selectable>().Info.Selectable && !world.FogObscures(x) && cond(x))