Replace bespoke save-queue dialog with tvxwidgets

[xxxserxxx]

There are a couple of places where it might be better to use tvxwidgets instead of the current bespoke stmps widgets. In particular, the dialog added as part of the save-queue-as-playlist feature is a lot of tedious code to correctly handle widget selection in the dialog. tvxwidgets also has some other "fancy" widgets, like proper progress bars, that stmps might have use for.

This ticket is specifically for integrating tvxwidgets to replace the save dialog, and can be closed when that's done. Other refactorings can be other tickets (lest this be a perpetually open ticket).

[xxxserxxx]

A second place is the spinner; tvxwidgets provides a spinner that's nearly identical to the one being used during the playlist load.

[xxxserxxx]

I looked into this in branch tvxwidgets and came to the conclusion that it was a dead-end. I don't remember why anymore; I think it was because tvxwidgets adds a dependency without dramatically simplifying the code.

I'm not looking into this anymore -- close it?