Unittest link generation for table of contents.

The logic was moved to a separate class and unittested.

Conflicts:
	sw/inc/ToxTextGenerator.hxx

Change-Id: I0e4475f5e2950cdfdfb07b89128c4ce1d6af3f22
Reviewed-on: https://gerrit.libreoffice.org/9609
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>