Removed IndependentAxes dead zone calculation

Thumbsticks have to always be considered as "circular", even if they are on independent axes on the hardware/driver side. Not doing so may expose clamping on the opposite axis when one axis if to its min or max value. This would results in a "sticky feel" when pushing a thumbstick to full X/-X or Y/-Y.
Restore IndependentAxes calculation and change range output

Restored dead zone calculation which was wrongly removed with previous commit.
Re-ranged the thumbstick output to match XNA (which exclude the dead zone from the final range).
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.
Fixed var typo.
Removed unused paramter.
Avoiding WP to try to use SharpDX.XInput.Gamepad constants.
Moving dead zone output range adjust to ApplyDeadZone.
Removed ConvertThumbStick which is not useful anymore.
Merge pull request #3305 from mrhelmut/develop

Excluding dead zone from final thumbstick output range