This allows requesting text/html, with falling back to plain text if
necessary.

Conflicts:
	libreofficekit/qa/gtktiledviewer/gtktiledviewer.cxx

Reviewed-on: https://gerrit.libreoffice.org/16377Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>
(cherry picked from commit af7cbebd)

Change-Id: Ie6d4e0e173311ba018553043b6a869abf193bf6f