Large refactoring of nwn-diamond PKGBUILD

* Modernize PKGBUILD (prepare, build, package functions)
* All DVD operations and archive extractions now happen in prepare()
* Patching only happens in prepare()
* Update nwn addons to use new github source location
* Quote all paths with variables
* Add default key file, instructions during install on how to use it
* Add key file to backup array
* Move all addon compilation to build() function
* Move all addon installation into custom _install_addons() function for
  clarity