SoC: GMail style conversation view for KMail

Changes:
* Clean-up of the painting code, making it easier to maintain.
* Refactorization of handling of date and time.
* The conversation display (MailView) now uses date and time when displaying e-mails.
* Some needed whitespace changes.

Status:
* Still nothing I'd show to my dad, but at least it shows what things will be like.

TODO in the near future:
* make the model sort by datetime of recent arrivals.
* make the model use the real Akonadi instead of these dummy classes.