Additional folder-centric settings

[jens-maus]

Conversion from TODO:
Every folder should be able to have its own appicon on Workbench, but the appicon would only appear when the folder contains new or unread mail - double clicking the appicon would automatically (1) uniconify YAM if appropriate and (2) select that folder. This would make life easier for users who have a multidrop account whereby they use to get mail addressed to sales@example.net, info@example.com, support@example.org, etc. but receive it all in the same mail account - the already existing mailing list support settings and some suitable filters would take care of everything else.

Charsets and salutation phrases should work on a per folder basis, too. In other words, they should be part of the "mailing list support settings", overriding the defaults set in the configuration.