ui: drop gtk2 support.

# gpg: Signature made Fri 12 Oct 2018 15:05:25 BST
# gpg:                using RSA key 4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/ui2-20181012-pull-request:
  ui: increase min required GTK3 version to 3.14.0
  ui: remove support for GTK2 in favour of GTK3

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>