A unique id has been introduced, in a similar way of what occurs in
Writer.

Change-Id: I7b2ef694867fb4184c4cfc616fe1c8f12da3b676
Reviewed-on: https://gerrit.libreoffice.org/36962Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan Holesovsky <kendy@collabora.com>