This requires distinguishing between 2 kinds of images: internal to
help (in helpcontent2/media/ and helpcontent2/res subfolder) and external
images from core repo (mostly icons) referenced in help

Change-Id: If1d639c255fb802c3ee7d7eab10fc0a84324f335
Reviewed-on: https://gerrit.libreoffice.org/31417Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>
Tested-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>
Tested-by: Jenkins <ci@libreoffice.org>