Showing a VC on top of the editor view now saves the selection.

This PR fixes [this bug](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/214).

There's some additional changes:
- Small code cleanup: removed unnecessary code.
- Added a fix to make the keyboard dismiss smoothly when the editor looses focus.
- Added a property to prevent on-time-setup code in `viewDidAppear` from executing twice.
- Added a VC to be able to test that the editor behaves correctly when hidden by something else.