Add initial support for writing XHTML markup as part of the HTML filter.
This already emits valid XHTML for hello world documents.

Times for 100 hello world inputs: 16032 -> 9957 ms is spent in ODT-load
+ export + close (62% of original).

Change-Id: I51a0a20985958fbc817c196d3a966e55dcb3f13f
Reviewed-on: https://gerrit.libreoffice.org/46567Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>