Make queue able to keep songs after playing them

[frestr]

This PR adds a new checkbox to the queue preferences - "Keep Songs".

When enabled, the queue will get a "CurrentColumn" and act more like the main song list. Songs can be added to the queue as normal, but they will be kept in the queue when playing them.

When "Keep Songs" is enabled, it's also possible to enable the "Prioritize Queue" checkbox. If the user selects a song in the main song list with this option enabled, then the player will switch to the queue after the song is done playing. This is the same as how the queue works now.

If "Prioritize Queue" is disabled, then the player will not switch to queue automatically (but rather play the next song in the main song list), and the user will have to click on a song in the queue to begin playing from it again. This makes it possible to occasionally ignore the queue without having to clear it.

It is quite rough around the edges, and some feedback would be very appreciated.

Some things to consider:

• There may be a better wording for the feature than "Keep Songs".
• The "Prioritize Queue" checkbox is not very pretty wrt. how it's disabled when "Keep Songs" is not checked. It's not very clear that the two are connected.
• It could perhaps be useful to have the queue remember its location when switching to the main song list, such that the queue automatically continues where it left off when the main song list is done.

Related issues: #1890, #2856.

[lazka]

Works nicely from quick test :)

There may be a better wording for the feature than "Keep Songs".

Nothing comes to mind, except "Remove Finished Songs" which is on by default, but I'm not sure that's better.

The "Prioritize Queue" checkbox is not very pretty wrt. how it's disabled when "Keep Songs" is not checked. It's not very clear that the two are connected.

We could have a "pause" button on the queue which makes things just ignore it as if it was empty in all cases.

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feel free to merge this and leave potential improvements for later

(I wont be cherry picking this to 4.1)

[frestr]

Thanks for the feedback!

We could have a "pause" button on the queue which makes things just ignore it as if it was empty in all cases.

I like that idea. Would be useful for the original queue behavior too.

feel free to merge this and leave potential improvements for later

All right. I'll finish it up and try to add some tests too.