TOX: Extract generation of tab stop attributes to its own class.

This allows us to pass through less parameters which are only used for
this functionality.

Conflicts:
	sw/inc/ToxTextGenerator.hxx
	sw/source/core/tox/ToxTextGenerator.cxx

Change-Id: I5a03cf8ec6d86d05f2a7f7771668585669c1e595
Reviewed-on: https://gerrit.libreoffice.org/9613
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>