Feature request: More advanced libAdwaita menu controls in "Menu Button" example

[TriVoxel]

The "Menu Button" example only shows basic menu options. I'd love to see the example expanded to include the fancy theme switching circles.

Something like this:

[sonnyp]

For this specific pattern we probably want to add it to libadwaita anyway (there was an issue IRC).

It would be good to show an example of custom menu widget 👍

[sonnyp]

Closing in favor of #118