CI: improvements

[bilelmoussaoui]

An issue to track the various improvements that would be nice to have as part of the CI

• Run clippy (needs some fixing probably in gir to avoid including unused stuff)
• Run non-sys build/tests with the highest vX_Y version feature possible
• Run -sys tests, see CI: add a simple github action #2 for details
  • gtk-sys fails currently
  • gdk-sys the tests takes way too long (not sure why?)
• Add a windows job
• Add a macos job
• Nightly docs
• Run generator & then git diff

[sdroege]

See #54 for the -sys tests problems.