Refactoring asynchronous operation code

[Stef Walter <stefw collabora co uk>]

I'm getting ready to do a big merge into seahorse. I've done some work
to refactor the way asynchronous operations work.

It'll now be more in line with GIO and other GNOME code.

I'm knocking the last few bugs out of the code. The branch is available
here:

http://cgit.collabora.com/git/user/stefw/seahorse.git/log/?h=async

Let me know if you'd like to look this over or have concerns before it
goes in.

Cheers,

Stef