This PR adds a `.visible_on_hover` helper method that can be used to
make an element only visible on hover.

I noticed we were repeating this similar stanza in a bunch of different
spots:

```rs
some_element
    .invisible()
    .group_hover("", |style| style.visible())
``` 

so it seemed like a nice thing to factor out into a reusable utility.

Release Notes:

- N/A