cc @SomeoneToIgnore

## Summary

Follow-up to [this
comment](https://github.com/zed-industries/zed/discussions/55352#discussioncomment-16961889).
This extracts the buffer header and breadcrumb rendering helpers out of
`element.rs` into a `header.rs` and mouse related things to `mouse.rs`

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:

- N/A