tradionally we do double/triple decker tab dialogs when there are a lot of tabs,
that option isn't available with native gtk3 notebooks.

Instead follow
https://developer.gnome.org/hig-book/unstable/controls-notebooks.html.en#controls-too-many-tabs
and when there are > 6 tabs position them at-left

Change-Id: I8e6ca78abeec3c1fa9f616c21f21fd7367be2ec9
Reviewed-on: https://gerrit.libreoffice.org/61487Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>