on_select for ListView

[Ameobea]

In my application, I'd like to have some explanatory text be displayed depending on the selected element in a ListView. However, it seems that no functionality exists to register a callback for this or even access the focused element of a given ListView (focus is private).

I'd be happy to implement this myself and create a PR. I'd add on_select and set_on_select methods to ListView (similar to that of SelectView) and perhaps expose a get_focused(&self) -> usize to get the currently focused element.

Would this be something that you'd be willing to accept?

[gyscos]

Absolutely, I'm not sure why I didn't include one already.
get_focused(&self) -> usize could be named focus, like its version in SelectView.

[Ameobea]

Resolved in 9894dd9