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
as a designer-only issue, can we stick this on a designer variable rather than window?