[FEATURE] add toggle_single_once action #637

Is your feature request related to a problem? Please describe.
I like using the toggle_single action to stop the music without interrupting the current song, but it's annoying when I forget to turn off single mode when I start the music again later.

Describe the solution you'd like
MPD already offers MPD_SINGLE_ONESHOT which automatically switches off single mode when the song finishes. It would be nice to have this MPD feature available in ncmpcpp.

Describe alternatives you've considered
An alternative way to achieve a similar result is to use a run_external_command action to invoke mpc single once, but this requires installation of mpc.