S-Pen button support

[rcheetah]

I read that after #8 we got support for palm rejection, which is great. But I really miss the other ideas. For now, the stylus button itself is completely useless (right?).

I would really love to see a freely configurable action for the stylus button (like long tab and high pressure tab). But front and foremost, maybe we can let the eraser tool be active, while the button is pressed? This would be so incredibly helpful, not switching tools all the time. The button is right at the stylus, and erasing while pressing it seems such an obvious behavior for a drawing app (seems, but sadly is not for many of them).

This would be so awesome, if there would be support for that!