...as had been done in the old build system (solenv/inc/extension_helplink.mk).
Especially for bundled extensions, this removes the need to compile the help
data per user on first start.

gb_Extension_add_helpfile(s) replaces gb_Extension_localize_help, and takes care
of all the steps (localization, compilation, inclusion in .oxt), even for the
en-US data (which was handled with additional gb_Extension_add_file calls
before).

Change-Id: Ie4bab66d3cad2b713780a23bf2606ca56cfff37f