Check that parameter `layout` to `layer()` works as expected in all layer functions.

[aphalo]

This is a new feature in 'ggplot2'. Does it get passed automatically to when 'ggpp' is running under 'ggplot2' >= 4.0.0 or is any change needed in stats and geoms?

• Add unit tests, one per layer function.
• Document in User Guide.
• Mention in articles.