[preview] Store downloaded files per JID
This concerns both URL Image Preview and OMEMO, downloading files into
configpaths.get('MY_DATA')) / 'downloads'.
I’m proposing to store downloaded files into separate (per JID) folders.
One potential use case for this would be an integrated overview of files/pictures within Gajim per contact, without the need for an additional index tracking individual file-to-contact relationships. This could be implemented wthin a page of the corresponding ChatControl for example.