Project panel loses the last scrolling position every time the user
hides/shows it. This PR fixes the problem.

The reason of the problem is that `UniformListScrollHandle`, which is
intended to store the scrolling position between redrawings, is only
used for ad-hoc autoscrollings to the list items, while the
`interactivity.scroll_handle` that is responsible for the scrolling
position, doesn't survive the project panel hiding.

How the problem looks:


https://github.com/zed-industries/zed/assets/2101250/7c7e3da6-9a9d-4f28-a181-ee9547349d4c

Release Notes:

- Fixed scrolling position restoration in the Project Panel.