Merge remote-tracking branch 'remotes/kraxel/tags/ui-20210521-pull-request' into staging

ui: add cut+paste support.
ui: bugfixes for spice and vnc.

# gpg: Signature made Fri 21 May 2021 13:50:46 BST
# gpg:                using RSA key A0328CFFB93A17A79901FE7D4CB6D8EED3E87138
# 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-20210521-pull-request:
  ui/gtk: add clipboard support
  ui/gtk: move struct GtkDisplayState to ui/gtk.h
  ui/vnc: clipboard support
  ui/vdagent: add clipboard support
  ui/vdagent: add mouse support
  ui/vdagent: core infrastructure
  ui: add clipboard documentation
  ui: add clipboard infrastructure
  build: add separate spice-protocol config option
  ui/spice-display: check NULL pointer in interface_release_resource()
  vnc: spelling fix (enable->enabled)

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