Latency calibration should mute menu music while open

[yeenbean]

It makes calibration a lot harder when two songs are playing at once.

[dtinth]

Thanks for the report!

I will put this on Hacktoberfest for now. :)

Steps to reproduce

1. Go to https://bemuse.ninja, Enter Game, and select a mode (e.g. Keyboard Mode). Menu music should start playing.

2. Open the Options screen (button located at bottom right corner). An options dialog should appear.

3. Click the Calibrate button. Another window should appear. While this window is open, the menu music should pause. Once the window is closed, the menu music can resume playing.