Add oapiGetSpecialFolderPath(PathRoot root) method to API

Leaving reasons aside, as a matter of fact graphic client developers currently have to manually load and process some files in some Orbiter's "special" directories (like "Textures", "Config" and so on), which either requires us to manually process main config file to figure out these paths (correct way, but time consuming), or just hardcode them (fast, but dirty way). There is a function oapiOpenFile() which opens file and takes care of directory mappings, but with it we're forced to use Orbiter's built-in function to work with the file, and they don't support some of the features that are supported by Orbiter itself - for example config file blocks like this one:
So having a function like that:
would help us a lot since we won't have to hardcode directories or manually do a mapping. Since there is oapiOpenFile() function which does the mapping I imagine it would be pretty easy to implement requested function.

Thanks!