...by storing the temporary HTML document in a location that can be accessed by
the browser running outside the Flatpak sandbox.  This reuses and extends the
mechanism already in place for the new HTML-based help in Flatpak mode (see
72b936d7 "Enable --help=html for flatpak").

This fixes <https://github.com/flathub/org.libreoffice.LibreOffice/issues/85>
"“Preview in Web Browser” does not work in Flatpak version".

Change-Id: I5f73fd89139ffe6b8ab0dc501154b4f054a0ae5c
Reviewed-on: https://gerrit.libreoffice.org/71570
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>