nightly-a8a73008-ls258

CI Report:

https://ci-tests.linuxserver.io/linuxserver/beets/nightly-a8a73008-ls258/index.html

LinuxServer Changes:

No changes

Remote Changes:

smartplaylist: Copy-paste-able list of playlists (#6404)

A simple QoL feature for splupdate - when wanting to update multiple
(not all) smartplaylists, it is not very convenient to copy-paste the
name from the output the plugin uses.

So for example issuing beet splupdate non_existent_list we get:
['playlist_name_1', 'playlist name 2', ...]. This output can't easily
be copy-pasted back to a shell command like

beet splupdate playlist_name_1 'playlist name 2'

This change handles quoting for shell using shlex.quote which makes it
easy to copy-paste several lists in a row (eg. with common prefixes).
Also the list of playlists is now sorted alphabetically prior to
printing on the shell!

Addition refactorings

A tiny fix below if not pretend: where .items() for dict-iterating
should have been used in the first place.