Rework the Akonadi engine in preparation for Contacts

- Fetch collections on demand only, should save us some startup time when loading the akonadi engine (no queries run until you actually use it)
- Link in KABC
- Some plumbing for Akonadi Contacts
- Renaming the keys for Emails and collections to:
EmailCollections to retrieve all emailcollections
ContactCollections to retrieve all contactcollections
Email-<id> to retrieve specific emails
Contact-<id> to retrieve specific contact (no workee atm)