Removed GTK+2.0 support

[Stef Walter <stefw collabora co uk>]

As I posted previously to the mailing list, I've removed GTK+ 2.0
support from gnome-keyring and seahorse.

It was getting to be a lot of work to keep support for the two, with
several inconsistencies and a bit of confusion. I didn't have time to
keep maintaining support for both versions of GTK+2.

Additionally seahorse depends on master of gnome-keyring (I'll be doing
a gnome-keyring release shortly).

Cheers,

Stef