Preview messages before emptying the bin, and notification bars that stay
out of the way.

- Tapping a message in the recycle bin now opens it, so you can read the
  whole thing before choosing between Restore and Delete forever. Deleting
  forever still asks for confirmation.
- The bar that appears at the bottom ("Message sent", "1 archived") is now
  see-through, so the message underneath stays visible instead of being
  covered. Its text carries a soft halo to stay readable against whatever
  shows through.
- On the inbox that bar used to float around mid-screen, because it was
  being stacked above the compose button. The compose button now steps
  aside while the bar is showing, so the bar sits at the bottom - and its
  Undo stays reachable rather than hidden behind the button.
- Clear SMS is on F-Droid, and the README now links to it.
