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
137e4e9251