Use space2ctrl for keys

[Chris Warburton]

Pull out xcape and xmodmap in favour of space2ctrl. We used to use it
many moons ago, I can't remember why we switched to xcape. In any case,
our xcape setup is definitely complicated and flaky, so might as well
try something simpler.

One important question is whether we can spot the reason for keys
becoming unresponsive. My current thoughts are:

 - Suspend/resume might be messing with things
 - Plugging/unplugging USB keyboards might be the cause
 - Plugging/unplugging external monitors might be the cause
 - Upgrading the kernel without rebooting might be the reason we need to
   rmmod/modprobe to fix the keyboard (which breaks the mouse)

It would be useful if we could either hook on to these
suspend/resume/plug/unplug events to run the keys command, or if we
could reliably spot when those events have happened and poll for
them. Either way, we shouldn't be running keys over and over.