settings: Support mouse and trackball accel profile

Support changing the mouse and trackball acceleration profile. This
makes it possible to for example disable pointer acceleration by
choosing the 'flat' profile.

This adds an optional dependency on gudev. Gudev is used by the X11
backend to detect whether a device is a mouse or not. Without gudev
support, the accel profile settings has have effect for mouse devices.

Trackball still uses the "strstr" approach, since udev doesn't support
tagging devices as trackball devices yet.

https://bugzilla.gnome.org/show_bug.cgi?id=769179