Improve Viewer HUD

[blairlyons]

• [ ] https://github.com/simularium/simularium-viewer/issues/122
• [x] https://github.com/simularium/simularium-website/issues/320
• [ ] https://github.com/simularium/simularium-website/issues/319
• [ ] https://github.com/simularium/simularium-planning/issues/123
• [ ] https://github.com/simularium/simularium-website/issues/217

Use case

As a user, I want the controls in the heads up display (HUD) to be non-intrusive but highly useful for navigating a trajectory.

Acceptance Criteria

• HUD button styles are consistent and buttons are easy to use
• Focus button usage is intuitive for users
• Hotkeys can be used to step through a trajectory
• HUD fades when not in use