gnome-keyring GTK+ 2.x support in nearing its end

[Stef Walter <stefw collabora co uk>]

Certain parts of gnome-keyring use GTK:

 * The gcr library (UI widgets)
 * The prompt tool

We're quickly accruing tons of #ifdefs from trying to support GTK+ 2.x
and GTK+ 3.x at the same time. This is hindering the code, and slowing
work on gnome-keyring.

Due to development time constraints, we'll probably have remove the GTK+
2.x support in the near future. I want to make sure gnome-keyring is
always buildable on a released mainstream distro (like Ubuntu or
Fedora). So removal of the 2.x code would happen after a mainstream
distro is released with GTK+ 3.x.

Cheers,

Stef