Switch to speed-based trackpoint acceleration

The current trackpoint acceleration uses deltas only but as described in https://who-t.blogspot.com/2018/06/observations-on-trackpoint-input-data.html deltas don't have enough dynamic range. We need to switch to a speed-based acceleration instead.

Bits needed

• large configuration range for the [-1, 1] speed mapping
  • acceptable at sensitivity 200
  • acceptable at sensitivity 128
  • acceptable at sensitivity 50
• Takes default sensitivity/hardware range into account
• replace the range attribute with a generic multiplier

Test branch: https://gitlab.freedesktop.org/whot/libinput/tree/wip/trackpoint-acceleration