OS X: correct tablet airbrush tangentialPressure to range -1..1

The driver sends values in the range 0..1, but we want the "center"
value to be 0.  This correction makes tangentialPressure consistent
between Linux, Windows and OS X.

Task-number: QTBUG-40469
Change-Id: Ia4aa777efdf015c2802b945f6ca7a8e442bbf5fc
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
QTabletEvent manual test: show buttons, pressure, eraser

- Show which button was pressed or released
- Render an ellipse proportional to pressure for each point drawn
- Different color for the eraser
- More complete output for each event
- Don't show mouse events by default, just as most tablet applications
  can now ignore mouse events on the drawing canvas.  But for the
  purpose of testing interleaving of tablet and mouse events,
  one can give the argument --mouse when starting this program
  to show them too, as before.

Task-number: QTBUG-39458
Change-Id: I5e03f1aa748be39d524bd6984ff5d66579787cf9
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
OS X: add buttons to tablet events

Task-number: QTBUG-39458
Change-Id: I112107e392bf3b55771039b72271fdf887e2e5db
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
OS X: correct tablet stylus rotation to range -180..180 degrees

When the user holds the Wacom Art Pen straight, rotation is zero;
when turning it counter-clockwise, it should have a negative angle,
whereas the driver sends a positive angle;
when turning it clockwise it should have a positive angle up to 180,
whereas the driver sends 360 going downwards towards 180.
These corrections make the angle reading consistent between Linux,
Windows and OS X.

Task-number: QTBUG-39570
Change-Id: I7a57cc1fb56d4f7128ca1add10aff2597f29c507
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>