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/SupportPowers/ParatroopersPower.cs

@@ -37,12 +37,12 @@ namespace OpenRA.Mods.RA
 		{
 			base.Activate(self, order, manager);
 
-			var items = (Info as ParatroopersPowerInfo).DropItems;
+			var info = (ParatroopersPowerInfo)Info;
+			var items = info.DropItems;
 			var startPos = self.World.ChooseRandomEdgeCell();
 
 			self.World.AddFrameEndTask(w =>
 			{
-				var info = (Info as ParatroopersPowerInfo);
 				var flare = info.FlareType != null ? w.CreateActor(info.FlareType, new TypeDictionary
 				{
 					new LocationInit( order.TargetLocation ),