[Patch] Improved pointer acceleration

[Simon Thum]

Hello list,

I've finished another iteration of my mouse pointer acceleration 
improvements. See here:
https://bugs.freedesktop.org/show_bug.cgi?id=8583

Besides minor stuff, I've improved (read: hopelessly overengineered) the 
velocity estimation algorithm, though by default it works like previous 
versions did.

Also, I've removed the Xinput cruft (runtime parameter adjustment) in 
favor of input device properties to take over that (when they're done).

A simple API for drivers is also included. This is intended to play nice 
in cases where special acceleration functions are desired, or 
driver-side mechanisms exist and would collide (such als old synaptics, 
but I bet there are more).

My suggestion would be to merge this version (after some review of 
course), and add runtime adjustment when the framework is ready. Which 
should be trivial.

Comments welcome!

-Simon