...that previously ended up in language-independent parts of installation sets.

The structure of that media/ tree doesn't allow to directly mis-use the existing
AllLangPackage machinery (which expects the language to be encoded in the first
pathname segment within the tree; and which is already mis-used for the
helpcontent2/AllLangPackage_html_lang.mk parts).

So introduce gb_AllLangPackage_add_files_for_lang that allows to specify the
language explicitly, independent of where it is encoded in the pathname (if at
all).  The underlying gb_AllLangPackage_add_file sets a
gb_AllLangPackage_ALLDIRS that is used by `make packageinfo`, which may need
further fixing by anybody actually using that target; see the mail thread
starting at
<https://lists.freedesktop.org/archives/libreoffice/2018-May/080242.html>
"Broken --with-help=html `make packageinfo`".

All files in $(SRCDIR)/helpcontent2/source/media/ must now explicitly be listed
in either helpcontent2/Package_html_media.mk (for the language-independent
files) or helpcontent2/AllLangPackage_html_media_lang.mk (for the language-
specific files).  Also note the two TODOs in
helpcontent2/AllLangPackage_html_media_lang.mk.

What is not quite right yet is that content from
helpcontent2/AllLangPackage_html_lang.mk and
helpcontent2/AllLangPackage_html_media_lang.mk is ending up in both per-language
helpcontent installation sets (as intended, via the instructions in
helpcontent2/CustomTarget_html.mk) and per-language languagepack installation
sets (which is unintended).  This needs to be fixed with a follow-up commit.

This is the core part of a commit spanning core and helpcontent2.

Change-Id: Ib29e52cf8a3ca7bcd234a8f6919c8eac6157cdbf