playSound() callback plus audio preload cleanup

[cpirich]

• playSound() now has a 3rd parameter (callback) - undocumented for student use. The callback function will be called when the sound starts playing. This will be used by hoc dance party so we can synchronize with the playing audio.
• Updated our playSound() unit tests to be aware of this change.
• Gamelab and Dancelab will now call register() on all preloaded sounds. This means that they will not only be preloaded into the browser cache, but will also be brought into memory so that we increase the likelihood that the sound will play immediately when the student presses the Run button.
• Moved the "preload asset list" portion of the level builder UI into the right place so that it appears for both droplet and blockly levels.

[cpirich]

@mrjoshida FYI

[islemaster]

🔊 🎉