ui: remove support for GTK2 in favour of GTK3

ui: remove support for GTK2 in favour of GTK3

GTK2 was deprecated in the 2.12.0 release with:

  commit b7715af2b31f47060cc5b4be930d16c13be93fa9
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Tue Dec 12 11:34:40 2017 +0000

    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>
It is thus able to be removed in the 3.1.0 release.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20180822131554.3398-2-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>