Add collections

[mrmn2]

PdfDing has the functionality to organize pdfs via tags. Introducing collections as an additional way to organize pdfs would improve the user experience greatly.

• collections should have their own section own the sidebar similar to tags
• while inside a collection filtering by search, tags, starred or archived should be applied the current collection
• collection section on the sidebar should be collapsible
• possibility to share collections with other users or the public

[LTsCreed]

Hi, just wondering how the feature implementation is progressing.

If you’re busy, I’d be happy to implement it myself

[mrmn2]

Update: It looks like I have more time to work on PdfDing in the next time, so I'll tackle this myself.