gtk: add and use DisplayOptions + DisplayGTK

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: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20180202111022.19269-5-kraxel@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>