In my testing on my local collection this increases hits among 25,362
calls from 18,814 to 19,352. This isn't necessarily substantial but I
don't have a very large collection myself, the memory savings to be
gained by improved odds of string re-use should have better odds with
larger music collections.

This feature is gated behind the hidden-by-default "Save Play Queue on
Exit" action so you'd need to add that to your toolbar for now if you
wish to test.

The hardest part was realizing we can't cast upwards to Playlist* before
we call the right operator>> to read the playlist data.

(cherry picked from commit c287962e)

(cherry picked from commit 6ee585d0)

In case of conflict in i18n, keep the version of the branch "ours"
To resolve a particular conflict, "git checkout --ours path/to/file.desktop"

(cherry picked from commit 90b7265e)

(cherry picked from commit fa039d5c)