More tweaks to SxS header processing

* Allow headers with only one label to be children of those with two (e.g. "Something about Appendix H" can be a child of "Something about Appendix G and Appendix H")
* When processing an appendix header, assume that any regtext citations are incorrect (e.g. ignore "2(a)" in "Appendix H - Model Form Clause 2(a)"