Expand search locations to a list of directories

[LegalizeAdulthood]

Currently if you're loading a formula, L-system, IFS, parameter set, colormap, Id will only look in one directory to find the file.

When installed to Program Files, Id will put all of it's data files there in a read-only location. The user will have their own set of data files in a write-only location that is distinct from Id's home directory. Rather than force the user to ping-pong back and forth between these two directories, there should be a list of locations that are searched for the relative files. This will also allow users to organize their data files in any way they like instead of forcing them to have them all in a single directory.

This implies that the id.cfg and directory parameters might need enhancing, or that the parameters allow you to add and subtract from the search list.