tablet: configurable eraser button
Some styli have the windows-style eraser button instead of the eraser at the back of the stylus. The button generates proximity events in the device, so the sequence on eraser button press is: stylus proximity out, eraser proximity in, and the reverse on release.
Users don't always want/need an eraser button but want that button to work normally, so we should make this configurable behaviour.
- we'll need libwacom support for tagging the affected styli
any stylus with that functionality needs to export nbuttons + 1 and return the right bits from
- the eraser button causes a proximity out event of the pen, then in the next evdev frame the proximity in, so we can't detect it as button press directly. We'll have to paper over this correctly.