randomize sprite positions when added from image file or from library (through 'choose from library' and 'surprise')

And start just creating assets to hand to the VM. Towards https://github.com/LLK/scratch-vm/issues/1577

Update calls to addSprite2 to use new API for adding sprites. Cleanup sprite3 representation for uploading a new sprite from an image.

Firefox and Safari both process mp3s as having audio/mpeg, and a variety of different audio/wav* possibilities.