Add abstractions for generic way of displaying *groupable* information

[terhechte]

Right now, Postsack only displays emails. However, most of the code is written in a way that it could easily be used on any kind of groupable information.

• The importer is abstracted away from the dabs
• The database is abstracted away from the guy
• The gui only uses a couple of currently hardcoded information from ps_core which can be extracted into a trait
  • The Field::all_cases and the fields that are available for grouping
  • Everything in the FilterPanel and the MailPanel. Those would need to be more generic
  • The importer flow (select mail folder)
  • The copy texts

It would still be some work, but in the end Postsack could be more generic and be applied to most groupable data.