schemas: add configuration options for pointing sticks

Add configuration options for pointing stick devices such as the TrackPoint. The options control speed, acceleration profile, and scrolling method.

This is heavily based on !21 (closed) by @Lawstorant, merely cleaning things up a bit. There is a companion MR for mutter that applies these settings and will be submitted momentarily. I had prepared these MRs a few weeks ago, but I never got around to testing (including building) them, partly because that seemed unusually involved and also risky from what I was told. Sorry about that! I thought it was better to submit than to let them rot.