Probably needs to be polished a bit more. Also, might require a key
binding that can set/clear the loop points in a more intuitive way.
For now, something like this can be put into input.conf to use it:
ctrl+y set ab-loop-a ${time-pos} # set A
ctrl+x set ab-loop-b ${time-pos} # set B
ctrl+c set ab-loop-a no # clear (mostly)
Fixes#1241.