lfrc - Added hotkey which converts selected videos (webm,mkv,mp4) to mp3 via ffmpeg

[TheYellowArchitect]

It ignores non-videos and videos which have no audio layer (ffprobe ftw). Moves them to the music folder, and notifies the user.

Change whatever you wish, as long the functionality remains the same (converting a song which is .webm ftw)

[LukeSmithxyz]

This seems like the kind of thing that should be a separate shell script, perhaps runable in lf with a shortcut, same with #1375.