Merge remote-tracking branch 'remotes/kraxel/tags/ui-20200114-pull-request' into...

Merge remote-tracking branch 'remotes/kraxel/tags/ui-20200114-pull-request' into staging

ui: add "-display help", gtk refresh rate.

# gpg: Signature made Tue 14 Jan 2020 10:01:14 GMT
# gpg:                using RSA key 4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>" [full]
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>" [full]
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>" [full]
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/ui-20200114-pull-request:
  display/gtk: get proper refreshrate
  ui: Print available display backends with '-display help'

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