[External Cursor API] Finalize public API surface

[Danielku15]

This item is about finalizing the final API surface we plan to ship for the external cursor API (backing track and external media).

It should make following things easy:

• Integrating an external media player
• Activating the built-in backing track
• Loading the backing track audio from a separate source (delayed and filling it)
• Switching between the player modes (thinking UI: allow the user to choose)
• Updating sync points