Support importing XSPF format playlists

[Denebet]

Steps to reproduce

Import an automatically saved playlist in xspf format (in home/XXX/.config/quodlibet/playlists) following another bug in quod libet which suddenly empties playlists

Expected Output

We should see, in the file browser that opens by clicking on "Import", the playlists in XSPF format in home/XXX/.config/quodlibet/playlists, and be able to select one and import it

Actual Output

Playlists in XSPF format do not appear

Test System

Which version of Quod Libet?

4.7.-1

Which operating system

Ubuntu 24.04

If it's audio-related, what back-end?

Additional Information

[declension]

Yes, this isn't supported currently so have converted this to a feature request.

Meanwhile, to help your current problem: you should be able to just cp / mv the playlist files into ~/.quodlibet/playlists and restart