Add support for the XDG Base Directory Specification

Please support the [XDG Base Directory Specification](https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html) on Linux/*nix systems.

Right now `dapr` dumps its files in `~/.dapr` which is not nice for several reasons.

If you think that splitting this data between _cache_, _config_ and _user data_ is not feasible to do right now, at the bare minimum put everything under `$XDG_DATA_HOME/dapr` (with a fallback to `~/.local/share/dapr`)

Users will be glad if applications stop putting junk under their home directories.

Thanks.