Map save / load rework

[brian]

The save dialog under windows adds the ".pmp" extension, under Linux (maybe other unix as well) it does not. So the map is saved with no extension under Linux. When you go to load the file it automatically appends the ".pmp" extension and there is no map matching that name/ext. so it blows up.

We also need to make the map saving/loading more mod friendly (per Philip's request).