Accessibility improvements to the micro:bit simulator. (#6113)

- re-structures the DOM so that tab ordering of simulator elements moves
  left-to-right and top-to-bottom.
- Makes the antenna element interactive using keyboard navigation, simple
  approach. Previously an invisible tab stop with no keyboard interaction.
- Add preventDefault to keyboard actions to prevent scrolling when setting
  values when the simulator is inside a scrolled container.

Author: glastonbridge