Handle relative links on image / text as data for a popup. Pick the images up
from <base directory>/<base name>/<relative url> as a start.

Change-Id: I9b6183d554e3792aa71dfffc19a671a0e4c302cc
Reviewed-on: https://gerrit.libreoffice.org/45601Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>