ui: increase min required GTK3 version to 3.14.0

ui: increase min required GTK3 version to 3.14.0

Per supported platforms doc[1], the various min GTK3 on relevant distros is:

  RHEL-7.0: 3.8.8
  RHEL-7.2: 3.14.13
  RHEL-7.4: 3.22.10
  RHEL-7.5: 3.22.26
  Debian (Stretch): 3.22.11
  Debian (Jessie): 3.14.5
  OpenBSD (Ports): 3.22.30
  FreeBSD (Ports): 3.22.29
  OpenSUSE Leap 15: 3.22.30
  SLE12-SP2: Unknown
  Ubuntu (Xenial): 3.18.9
  macOS (Homebrew): 3.22.30

This suggests that a minimum GTK3 of 3.14.0 is a reasonable target,
as users are unlikely to be stuck on RHEL-7.0/7.1 still

[1] https://qemu.weilnetz.de/doc/qemu-doc.html#Supported-build-platforms

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20180822131554.3398-3-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>