ui/gtk: move struct GtkDisplayState to ui/gtk.h

Want place gtk clipboard code in a separate C file, which in turn
requires GtkDisplayState being in a header file.  So move it.  No
functional change.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20210519053940.1888907-1-kraxel@redhat.com
Message-Id: <20210519053940.1888907-9-kraxel@redhat.com>