Fixed some a11y issues

As reported in [NOTA-99](https://notalib.atlassian.net/browse/NOTA-99)

Several changes were made:

  - We no longer continuously focus on the "pause" button when playing - this prevented "flicking"/tabbing through elements while playing
  - The book-cover images are now ignored by screen-readers (as the title is already being read to them)
  - I've added `aria-label` to the player controls, which now should be more accessible when using screen-readers
  - Different loading messages (loading book, loading sound, etc.) are now `role="alert"` meaning that they get read when they're shown
  - The "sample text" in the settings dialog (used to see font-type and font-size changes) were read aloud before - not any longer
  - The playback rate, radio-buttons now have more meaningful labels, such as "slowest", "slower", etc.

I had some troubles with iOS VoiceOver when trying to remove books by clicking the `x`, the device would respond as if there was no clickable element underneath, the same applies to the bookmarks section (I can't click the `...` icon)

Test branch available here: http://test.m.e17.dk/a11y

This should also be merged into master at some point