* Added separate file for the type of algorithm: `algorithm.vala`
  This should facilitate adding other encryption types.
* Added SSH-specific errors: `errors.vala`. Just a stump right now, but
  we should be able to expand that when the time comes.
* The dialogs no longer use Seahorse.ObjectWidget/Seahorse.Widget. (In
  the long term, we'll probably deprecate these)
* An Operation is an object, which provides us a convenient closure.
* Added a lot more comments.
* Some types of base classes should've been nullable, but weren't.