This PR changes the API for the `ListItem`'s `end_hover` slot so that
whatever is in there is always part of the flex stack, as opposed to an
absolutely-positioned element. Additionally, I'm also improving the API
for swapping content between the default state and the hovered state
(e.g., list items where by default we render X, but when you hover, we
show something else). Lastly, I'm adding buttons to some Git picker
items that were only previously available through modal footer buttons.

Release Notes:

- N/A