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.Editor/BrushTool.cs

@@ -109,7 +109,7 @@ namespace OpenRA.Editor
 			s.Chunks.Clear();
 		}
 
-		CPos FindEdge(Surface s, CPos p, CVec d, TileReference<ushort, byte> replace)
+		static CPos FindEdge(Surface s, CPos p, CVec d, TileReference<ushort, byte> replace)
 		{
 			for (;;)
 			{