#67 Define accessors for special paths

[Andrei Kapustin]

There are "special" paths in every OS - e.g. "user's home directory", "temp directory", "directory for shared settings", "desktop's start menu directory", etc. The first two can now be obtained via dedicated FileSystem services, the rest of them cannot.

Define a SystemPaths utility class that will provide accessors for all of these system-special paths in a uniform manner.