FEAT: POP3 leave-on-server and UIDL tracking

[andrinoff]

Is your feature request related to a problem?

POP3 support should offer leave-on-server with proper UIDL dedup.

Describe the solution you'd like

Add a leave-on-server option and track UIDLs so messages aren't re-downloaded or deleted unexpectedly.

Describe alternatives you've considered

Download-and-delete only.

Additional context

See backend/pop3.

[floatpanebot]

Thanks for the suggestion!

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