The nsIRelativeFilePref interface is a wrapper for an nsIFile and
and a directory service key. When used as a pref value, it stores a
relative path to the file from the location pointed to by the directory
service key. The path has the same syntax across all platforms.
@see nsIPrefBranch::getComplexValue
@see nsIPrefBranch::setComplexValue
file
The file whose location is stored or retrieved.
relativeToKey
A directory service key for the directory
from which the file path is relative.