Refactor asynchronous operation code in seahorse.

 * This is a very large commit.
 * We use GIO style async now instead of SeahorseOperation.
 * Makes things easier to hack on because it's more consistent with
   other GNOME software.
 * More predictable cancellation and error code paths.
 * Some other cleanup here too. When rewriting entire functions to
   refactor the async code, I cleaned up some other things too.