Fixed IDisposable implementation and usage.

- Implement IDisposable interface correctly, with sealed classes where possible for simplicity.
- Add using statement around undisposed local variables.

OpenRA.Game/Support/PerfHistory.cs

@@ -105,7 +105,7 @@ namespace OpenRA.Support
 		}
 	}
 
-	public class PerfSample : IDisposable
+	public sealed class PerfSample : IDisposable
 	{
 		readonly Stopwatch sw = Stopwatch.StartNew();
 		readonly string Item;