[Input] GamePad and Joystick classes should be sealed

Source/OpenTK/Input/GamePad.cs

@@ -32,7 +32,7 @@ namespace OpenTK.Input
     /// <summary>
     /// Provides access to GamePad devices.
     /// </summary>
-    public class GamePad
+    public sealed class GamePad
     {
         internal const int MaxAxisCount = 10;
         internal const int MaxDPadCount = 2;
@@ -40,6 +40,8 @@ namespace OpenTK.Input
         static readonly IGamePadDriver driver =
             Platform.Factory.Default.CreateGamePadDriver();
 
+        private GamePad() { }
+
         /// <summary>
         /// Retrieves a <c>GamePadCapabilities</c> structure describing the
         /// capabilities of a gamepad device.

Source/OpenTK/Input/Joystick.cs

@@ -33,11 +33,13 @@ using System.Text;
 
 namespace OpenTK.Input
 {
-    public class Joystick
+    public sealed class Joystick
     {
         static readonly IJoystickDriver2 implementation =
             Platform.Factory.Default.CreateJoystickDriver();
 
+        private Joystick() { }
+
         public static JoystickCapabilities GetCapabilities(int index)
         {
             return implementation.GetCapabilities(index);