Extract TOX text generation to its own source file

The separate source file will make it easier to unit test the functionality

Also: move some constants to tox.hxx to have them available in the new source file

Conflicts:
	sw/source/core/doc/doctxm.cxx

Change-Id: Ib0369cc4875043829f55d30a549997560a38cc35
Reviewed-on: https://gerrit.libreoffice.org/9607
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>