Add "emulateWheel" option for trackpoint configuration

This option enables scrolling while holding the middle mouse button on a Thinkpad. I've already tested the changes on my X201s / X220  and it works fine.

The configuration comes from [ThinkWiki](http://www.thinkwiki.org/wiki/How_to_configure_the_TrackPoint#Scrolling)

Unfortunately, I couldn't find a way to activate the changes on a running system. So the user has to reboot  the machine (or restart xserver) to make the changes available. The only way to configure ```emulateWheel``` is through xserver configuration and this config is only read when xserver starts.