There was only horizontal and vertical previously, so keep things simple
and talk about the BT and TB version of vertical, not mentioning the
LR/RL aspect.

Also rename the textdirection widget, so it's unique not only within the
tab page, but inside the dialog, so we can have uitest coverage for
this.

Change-Id: Ie396898fde03aca6cd37a29f049099fa4b2c5fc0
Reviewed-on: https://gerrit.libreoffice.org/67789Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins