libinput and tablet tool eraser buttons

This is, to some degree, a followup to this 2014 post . The TLDR of that is that, many a moon ago, the corporate overlords at Microsoft that decide all PC hardware behaviour decreed that the best way to handle an eraser emulation on a stylus is by having a button that is hardcoded in the firmware to, upon press, send a proximity out event for the pen followed by a proximity in event for the eraser…