Add QAPI DisplayType enum, DisplayOptions union and DisplayGTK struct.
Switch gtk configuration to use the qapi type.

Some bookkeeping (fullscreen for example) is done twice now, this is
temporary until more/all UIs are switched over to qapi configuration.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 20180202111022.19269-5-kraxel@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>