permissions: set album art permissions

[Duncaen]

Description

This change makes the permissions plugin set the album art file permissions to the configurable file permissions whenever the set_art event is triggered.

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)

[sampsyo]

Looks good overall; thanks!! I have a couple of comments:

• It would be great to reduce the duplication with the fix method—in particular, the logging and such. Can we make a new helper method to cut down on the copied code?
• Could you please add a changelog entry? (I don't think this needs other documentation.)
• I'm also not sure whether the tests would be easily extended to cover album art, but it's worth a quick glance to see.

[wisp3rwind]

This is looking good to me, what do you think @sampsyo ?

[sampsyo]

Thanks for the ping; yes, this all looks great! Nice work. Looks like it needs an update and then should be good to go.

[Duncaen]

Thanks, rebased and fixed the change log conflict.