Allow the flat acceleration profile for touchpads

I've been using libinput with a similar patch for years now, and @tbrodbeck's issue #283 (closed) from a couple months ago has prompted me to flesh it out as a PR.

As @benrki put it in #283 (closed), flat acceleration is nice on touchpads for the same reasons it's nice on mice. For me, it removes a certain layer of cognitive overhead—instead of thinking about how fast I'm moving my finger, I only have to think about where I'm moving it.

I prefer flat acceleration on every touchpad I've used, including that of a T480s, a MacbookPro11,5, and external touchpads like the Magic Trackpad 2. It takes time to get used to any new acceleration curve, but since making the switch, touchpads have felt a lot more fluid and accurate, and I've become significantly faster at touchpad-intensive tasks like surfin' the net.

@whot, I understand that you haven't yet experienced flat acceleration nirvana, but I know there are users who would appreciate the option :)

I'm not too familiar with libinput's structure, so I've marked this PR as a WIP.