The about window currently is a "Normal" window, which is suboptimal on
tiling window managers.

This changes it to be a floating window, causing less disruptions to
opinionated WM layouts.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed the "About Zed" dialog being a full window instead of a floating
window.