Improve stop button usability

First I removed the logic to adapt the StatusFragment to state ErrorNoNetwork. We no longer need this as we're displaying NoNetworkFragment anyway in this case instead of a modified StatusFragment.

Then I added a question mark button to the side of the stop button. Clicking it brings up a dialog with some info about what stopping the mailbox means. Hitting the stop button itself brings up a confirmation dialog now with the same text.

Here's how it looks:

Closes #178 (closed)