Add support for touch events to violent theremin demo

[normanr]

It would be nice to be able to use the demo in a touch environment (eg: phone or tablet). (Not required: sound could be continuous after a 'start/down' event until an 'end/up' event is received)

[chrisdavidmills]

I agree that this would be nice.

For reference:

• Demo repo: https://github.com/mdn/violent-theremin
• Live demo: https://mdn.github.io/violent-theremin/

[normanr]

(side-note: the event listeners for triggering init should be removed in init, otherwise using the keyboard causes init to be triggered multi times, and DOMExceptions when trying to remove the startMessage that has already been removed).

[chrisdavidmills]

Issue moved to mdn/content mdn/webaudio-examples#126 via ZenHub