New auto-shutdown feature

[nokitron]

Hello, MusicBee. Firstly, great job on the big update. I really enjoy the new features.

There is one thing that always bother me about auto-shutdown. I use it each time and it is so convenient. So far we have elapsed minutes, end of track, and end of playlist.  I wonder if new choice is possible, which is determined by how many track you want before it triggers the action. Let's just say I want Musicbee to stay active for next 5 tracks before it shuts down. It would be very wonderful addition to the app and makes MusicBee becoming more powerful.

Keep up the good work, we all are fans of MusicBee