gnome-keyring REVIEW: GcrGnupgCollection implementation

[Stef Walter <stefw collabora co uk>]

Here's a patch that needs review.

This adds GcrGnupgKey and GcrGnupgCollection. The first is a basic key
object that has some properties which expose attributes of a gnupg key.
The second is a collection which loads gnupg keys.

This is just the basic support. It's built around the gnupg
--with-colons support, and doesn't require intermediate libraries like
gpgme.

https://bugzilla.gnome.org/show_bug.cgi?id=647885

When something doesn't yet go into the public gcr API, we prefix it with
_gcr_ instead of gcr_. This initial work doesn't go into the public api
(at least not yet), and we'll build a high level selector/chooser on top
of it which will go public.

To play with it use gcr/tests/frob-gnupg-selector

Looking forward to your review and comments.

Cheers,

Stef