drafts: remember In-Reply-To, References and Date headers as well

refs #602
drafts improvements

- better "need-to-save" detection
- autostore in cache, not temp
- fix file permissions
- delete sent drafts
- use QLatin1Spam
- more comments
- warn user that attachments are not saved
drop slotSelectionChanged

contradicts with explicit blocking of Ctrl or Shift click
GUI: reduce the interval in which "today's messages" are recognized as today's