Add checkbox to stop player after queue is empty

[frestr]

Fixes #43

Although the "Queue Only" plugin has this as a side effect, the effect of always adding songs to the queue may not be desired. This addition instead adds a checkbox - "Stop After Empty" - next to the "Random" checkbox that has no other side effects than stopping the player after the queue is empty, instead of continuing to play a song from the song pane.

[declension]

I think Stop Once Empty reads a bit better than Stop After Empty

[frestr]

I agree. I based it on "Stop After This Song" under Controls, but the wording makes more sense in that case.

[declension]

Looks reasonable in general, and works for me - thanks.

Part of me still thinks this can / should be solved as a play order plugin / configuration for the queue itself, but that's a much bigger piece so maybe best to get this in for now.

[frestr]

I thought about making it a play order plugin, but that doesn't make much sense the way I see it, as it doesn't really have that much to do with the play order.

I think having a separate configuration window for the queue would be nice, especially if we later want to add even more queue options not relating to the play order. Perhaps we could add a new "Queue" tab in the File->Preferences window?

[declension]

Yep I think all it's a bit complicated conceptually and UI-wise both now and maybe forever. Essentially I wanted to provide a way to abstract the PlaylistMux better, and also unify the "Random" with Shuffle plugins (why can't the queue be shuffled in a weighted way, or whatever).

That way, this and the Queue only plugin could live somewhere a bit more meaningful. Though of course that then makes it more complicated, if slightly neater / more powerful.

...but again this is a whole separate piece really.

@frestr if you can make that text change, then assuming @lazka doesn't have anything I think it's good to go.

[declension]

Thanks

[lazka]

Thanks