Finish agi::Path so we can actually use it, our current path system is brutal...

Finish agi::Path so we can actually use it, our current path system is brutal and I needed this for libmedia.  I'll be adding OS X support shortly: someone else will have to figure out what to do with windows it's completely trivial see unix/path.cpp and check libaegisub/path.h for the return values.

Originally committed to SVN as r5315.