Add WoPPack to upstream NetRadiant

[robo9k]

Once it is somewhat stable, WoPPack should be included in NetRadiant, so people can get it with the regular distributions of NetRadiant (instead of being bundled with World of Padman).

This would just require adding a line like the following to NetRadiant's download-gamepacks.sh:

+pack WoPPack         proprietary git    https://github.com/robo9k/WoPPack.git