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.
