[FR] Update PropertiesFile to allow use of XDG_CONFIG_HOME on Linux

Maybe it’s just an old bit of the code base, but shouldn’t PropertiesFile allow the use of XDG_CONFIG_HOME on Linux rather than ~? I guess commonToAllUsers should be able to reference XDG_CONFIG_DIRS too.

Suggestion to avoid backwards compatibility issues: could add a new .linuxShouldUseXdgConfig property or some such?

Regards,
Andrew