Add .visible_on_hover helper method (#3639)

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

crates/ui2/src/ui2.rs

@@ -21,6 +21,7 @@ mod selectable;
 mod styled_ext;
 mod styles;
 pub mod utils;
+mod visible_on_hover;
 
 pub use clickable::*;
 pub use components::*;
@@ -30,3 +31,4 @@ pub use prelude::*;
 pub use selectable::*;
 pub use styled_ext::*;
 pub use styles::*;
+pub use visible_on_hover::*;