Follow XDG Base Directory Specification

[phanirithvij]

Thanks for this great local-first time tracker software!

I have a small improvement to suggest, can the ~/_timetagger be moved to ~/.config/timetagger or ~/.local/share/timetagger?

Here's the specification for that which many of the programs do try to follow to avoid cluttering up the home directory of the user, https://specifications.freedesktop.org/basedir/latest/

[phanirithvij]

I've found something better to handle it cross platform, https://github.com/tox-dev/platformdirs

I will propose a PR soon.