Moved thumbstick dead zones constants to GamePadThumbSticks.ApplyDeadZone(...)
Removed double dead zone computation when calling IsButtonDown or IsButtonUp
Added new internal ctor to GamePadThumbSticks to handle correctly dead zones before rounding/clamping values (XNA compliant)
Moved all dead zones computations to GamePadThumbSticks ctor instead of manually calling ApplyDeadZone.
Made ApplyDeadZone private so that it is not mistakenly called outside GamePadThumbSticks ctor.