Add macOS support

[mentaljam]

[drkvogel]

I'd like to have a go at this, though I'm a bit of a noob at Rust (used to do a lot of C++...). Any tips?

[drkvogel]

Or has this library been superseded by something else?

[mentaljam]

Hello and thanks for your reply! I ported this library for another (currently abandoned) project. But it seems useful to someone, as it continues to be downloaded on crates.io :)

If you want to implement support for macOS, you can start from the original QStandardPaths and port the appropriate part of the code. I used version 5.x, but I think you can start with version 6.x.