Fix: dynamic playlists with flatpak build

[maxmitti]

Fixes #91

Starting and stopping dynamic playlists with the flatpak build works.
However, when Cantata is quit while the dynamic playlist is running the cantata-dynamic script keeps running and can only be stopped by killing it externally.

[lokke3]

IIRC the behavior of the dynamic playlist continuing to run after quitting Cantata has existed for a long time. Not sure if that is actually a bug.

[nullobsi]

IIRC the behavior of the dynamic playlist continuing to run after quitting Cantata has existed for a long time. Not sure if that is actually a bug.

Ideally it would start/stop with Cantata. I do not think it translates well to Flatpak (it will show a "running" indicator even when it is not running).

[nullobsi]

Regardless, I think a partial fix is as good as any. I hope that the changes will work on the Flathub build too.

[maxmitti]

IIRC the behavior of the dynamic playlist continuing to run after quitting Cantata has existed for a long time. Not sure if that is actually a bug.

It seems intended, especially because there is also a systemd service file: https://github.com/nullobsi/cantata/blob/main/playlists/cantata-dynamic.service

Regardless, it is buggy with the flatpak, because a newly started instance can’t communicate with the cantata-dynamic instance from the previous instance.
I also think that stopping the script when Cantata (in flatpak) is closed is the easiest fix for that.

[maxmitti]

I had another idea: The dynamic playlist socket could be located in the XDG runtime directory instead, which should be accessible by newly started instances as well.

Nevermind, I just realized that the communication happens via mpd channels, so that is not the issue.