Simple XInput 2 client for use in croutonpowerd and croutonwheel.

Low priority.

This is a simple Xinput 2 client, largely inspired from the existing `xinput test-xi2`. The main difference is that it listens on the root window, so no need to do tricks to hide the window that `xinput test-xi2` creates. xinput is able to do the same now, but this has only been recently merged into the git tree (http://cgit.freedesktop.org/xorg/app/xinput/commit/?id=39ac9d85f464ca3195e8b3b2d3f6c3188f83e25e), so it will probably not appear in distributions anytime soon.

So, we use it for 2 purpose.

1. In `croutonpowerd`. Instead of polling `xprintidle`, we listen for input events (I believe this is completely equivalent: is there anything but input events that would reset the idle time?). This has 2 advantages:
 - With `xprintidle`, we poll every 2 seconds. That means that once the screen is dimmed after inactivity, it can take up to 2 seconds for the screen to come up again. With `croutonxi2event`, we react immediately, and the user does not lose precious seconds of productivity after going for a coffee break ,-)
 - `xprintidle` is an orphan package, no one is maintaining it anymore (the "official" website is not accessible): better not to rely on it...
 - It's a bit tricky to monitor for screensaver status at the same time, but what I wrote works fine. There is a little "limitation" where the system could potentially go to sleep after 4'45" instead of 5', but I don't think it's too serious. The logic to make it sleep after 5'15" is slightly more complicated, and pings the power daemon twice as often, which is why I chose the former.

2. In `croutonwheel`:
 - Since we know that the output will be parsed by awk, we print the whole event information on a single line. And we only print relevant events.
 - No need for the hack to hide the xinput window (the hack does not work perfectly: you still see the window for a fraction of a second).