Doomsday Engine

Refactor: Rewrote the help text database (dd_help)

dd_help.c was rewritten to use C++, libdeng2, and more efficient
data structures. It now also supports full Unicode text.

Updated the module's internal C API with modern conventions in mind
(the API itself was left unchanged).

Important note: the source data (cphelp.txt, conhelp.txt) is now read
directly from within the PK3s using libdeng2. Thus it is no longer
necessary to deploy them as "startupdata" in the installation.