Until now, it has been necessary for tablet-oriented applications which
care about multi-button styli to reject each tablet event and wait for
the mouse event in order to know which buttons are pressed.  This patch
adds the new API and also the X11/xcb implementation.

[ChangeLog][QtCore][Tablet support] Added buttons to QTabletEvent

Task-number: QTBUG-39458
Change-Id: If2c9ec1ceacc1700a82686c5fc6f568f9111055a
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>