ui: deprecate use of GTK 2.x in favour of 3.x series

The GTK 3.0 release was made in Feb, 2011:

  https://blog.gtk.org/2011/02/10/gtk-3-0-released/



That will soon be 7 years ago, which is enough time to consider
the 3.x series widely supported.

Thus we deprecate the GTK 2.x support, which will allow us to
delete it in the last release of 2018. By this time, GTK 3.x
will be almost 8 years old.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20171212113440.16483-1-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>