[SDL] runtime data path

[Tim Goya]

The key is that we (SuperTux) don't do it based on current *working*
directory, but current *executable* directory, obtained through
PhysFS.  It's not as clean as it could be since it differentiates
between Unix and Windows when both can be handled by the same code.
Hmm, I should go fix that.  It also does funny stuff with char * where
std::string is clearer and less error-prone.  I should fix that too.

Tim