Adds the ability to have some effect run when a selection changes in a
picker.

If the `PickerDelegate` implements something other than `None` for
`selected_index_changed` then each time the selection changes it will
run that effect.

For example:

```rs
impl PickerDelegate for PromptManagerDelegate {
    //...

    fn selected_index_changed(
        &self,
        ix: usize,
        cx: &mut ViewContext<Picker<Self>>,
    ) -> Option<Box<dyn Fn(&mut WindowContext) + 'static>> {
        Some(self.prompt_manager.set_active_prompt(ix, cx))
    }

    //...
}
```

This isn't currently used in any picker, but I'm adding this to allow
the functionality we intended for the prompt library, we're changing
selections, activates a preview in the right column.

This will be useful for building any sort of UI where there's a picker
on the left and a preview on the right, such as a UI like them
telescope.

Release Notes:

- N/A