Implement initial support for XDG [1]

Introduce getFontForgeUserDir(arg) where arg is one of Cache, Config, or
Data.  Replace every instance of getPfaEditDir(buffer) with
getFontForgeUserDir(Config).

[1] http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
Merge branch 'xdg_base_dir_spec' of https://github.com/holomorph/fontforge into holomorph-xdg_base_dir_spec