This adds the public API to configure an eraser button on a tablet tool
to emulate a normal button. In DEFAULT mode the eraser button will
simply do whatever it does by default (i.e. toggle to eraser).
In BUTTON mode the eraser button will be converted to a regular tool
button event, with libinput handling the underlying proximity event
madness.
Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1218>
