Merge remote-tracking branch 'remotes/kraxel/tags/pull-ui-20170614-1' into staging

ui: prefer gtk3 and sdl2, various fixes.

# gpg: Signature made Wed 14 Jun 2017 08:54:22 BST
# gpg:                using RSA key 0x4CB6D8EED3E87138
# 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/pull-ui-20170614-1:
  spice: don't enter opengl mode in case another UI provides opengl support
  sdl: prefer sdl2 over sdl1
  gtk: prefer gtk3 over gtk2
  spice: Use proper enum type for kbd led state
  Improve Cocoa modifier key handling

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