Initial stab at #165, $prefix per-user directory

Hey TTimo,

Here's a changeset which introduces $prefix_win32 to the .game XML file. The logic here is to use ~/.${prefix} on Linux and Mac, and \\My Document\\My Games\\${prefix_win32} on Windows. I've added ${prefix_win32} to the generated .game file for Quake2World.

I have no way to compile or test this code, but I was hoping it'd help with #165.