Too many Audiocontexts fix, fixes #2211

Keeping an Audiocontext on the window now, so that when we create a new app in the designer it doesn't try to create a new Audiocontext every time (and run out)

Unfortunately once you create an audiocontext, you cant 'clear' it - so this seems like the way to get around that.

STR:
* Open designer
* Hit "New App" 6 times
* Add a Button - it still play sound and there is no Audiocontext console error
rewrite suggestion if we're using window:
```
var audioContext = window.AudioContext || window.webkitAudioContext; // shadows window.audioContext now
if(audioContext) {
  if(!window.audioContext) { // sticking with JS var vs. object notation conventions
    window.audioContext = new audioContext();
  }
  this.audioContext = window.audioContext;
}
```