add button information to QTabletEvent

The Krita authors have reported that they need to reject each QTabletEvent and wait for the mouse event that follows, in order to find out which buttons are pressed. The reason is that tablet and stylus buttons are sometimes used during the drawing process itself, not just as mouse buttons for conventional UI elements. So it makes sense to have the buttons in the event, since this information is already known when the event is created.

The trouble with rejecting tablet events is that the queue is cleared at that point, so some events will be discarded, and the result is not very smooth drawing.