Improve accessibility

Various small accessibility improvements. For more details please
see the individual commit messages.

Two major improvements are dependent on improving used libraries:

* ECSlidingViewController - I created https://github.com/edgecase/ECSlidingViewController/pull/127
  for this. Shall I also fork ioctocat/ECSlidingViewController and submit
  the same change there, or change it inline in the dennisreimann/ioctocat
  repository?
* TTTAttributedLabel - each link and non-link segment should be a separate
  accessibility element. The EventCell should then also present all its
  elements separately for this to have effect. I hope to work on it
  the next weekend.