Select*KeyFragment rewrites

- moved all specific Public/Secret logic from the Adapter class into
  inner subclasses in the Fragments
- more versatile status display ("revoked", "expired", "can certify"...)
- applied view holder pattern
- query logic, including subqueries, moved into provider classes

Closes #375
Merge pull request #550 from Valodim/select-fragment

Select*KeyFragment rewrites