Add play/pause support for 3rd screen

[jbphet]

We've discussed adding play/pause capability to the "Spin" screen (screen 3) during some of the recent design meetings. I just checked the design document, and in the "Meeting Notes" section for November 26, 2024 it says "Add PlayPauseButton to Continuous Spin scene" in the "Decision" column of the table. So, I think it's time for this feature to be added.