Switch to Meson

[All Aboard The Meson Future Hype Train](https://medium.com/elementaryos/all-aboard-the-meson-future-hype-train-2b6c478b6b9e) explains that elementary OS will be switchting over from CMake to Meson. This probably means we'll also need to update this project, since it's used in the [getting started documentation](https://elementary.io/docs/code/getting-started)

> you can expect our documentation to be updated as soon as the first preview builds of Juno become available to developers.

From: [All Aboard The Meson Future Hype Train](https://medium.com/elementaryos/all-aboard-the-meson-future-hype-train-2b6c478b6b9e)

I thought it might be interesting to have an issue for discussions and to track progress.

ToDo:
- [ ] Adapt rules file

Also see: https://github.com/elementary/website/issues/1885