Adhere to XDG Desktop Specification and use the environment variable XDG_CONFIG_HOME to determine location of configuration files on *all* platforms. If it is unset or empty resort to platform-specific defaults. On Windows and OS X, `wxStandardPaths` provides correct defaults (equal to the previous hard-coded paths) but on Linux and other Unices `~/.config/appinfo` would be better than `~/.appinfo`. This means we treat those platforms as a special case. It also means that we may need to fall back to the legacy location if it exists but the new location doesn't.