Related: tdf#120371 a scheme to provide multi-level notebook tabs

Change-Id: Ib92b5e63aa6457f5df8e51fe5404d2341aae5ffb
Reviewed-on: https://gerrit.libreoffice.org/64767
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit bf379a10)
Reviewed-on: https://gerrit.libreoffice.org/64848