Add support for the original SeriousSam games

Hi,

These changes are part of my effort to bring the original Serious Sam games to Debian, I uploaded the engine part on mentors.debian.net : https://mentors.debian.net/debian/pool/contrib/s/serious-engine/

I know that the changes here in runtime/gdp_launcher_base.py are not perfect, the fact is that the "symlink_into_dot_directory" directive in the "launch system" only handle directories, not files :/

UPDATE: found a way by reading serious-engine sourcecode

What are your thoughts about all this ?