Simple XInput 2 client, for use in croutonpowerd and croutonwheel.
Make croutonpowerd use croutonxi2event:
 - No need to use xprintidle (orphan package)
 - Ping immediately on keypress (instead of having to wait up to 2 seconds)
 - Polling time can be reduced significantly
Make croutonwheel use croutonxi2event:
 - Parsing logic is simplified (output on a single line).
 - Less "garbage", croutonxi2event only outputs XInput events.
 - No hack needed to hide xinput window.
Rewrites part of xi2event.c that were directly copied from xinput.
Still looks very similar as this is essentially the only way to do things.
Fixes after review.
Better xinput mask allocation.
croutonpowerd: Test if $xi2pid is empty before checking the process status.
Merge pull request #331 from drinkcat/newxi2

Simple XInput 2 client for use in croutonpowerd and croutonwheel.