Some more studio cleanup

A little more cleanup as I try to understand Studio code.

(1) We were setting some fields on Blockly.Blocks (which is otherwise composed entirely of actual blocks). I moved this to local variables.
(2) NthStartingSkin seems really awkward to me. Instead I created a list of startAvatars that is reordered according to firstSpriteIndex.
(3) We had a circular dependency (studio.js requires block.js requires studio.js), that I'm surprised browserify is okay with.  I removed block's dependency on studio by having studio set startAvatars (which will change per level).
That would be kind of fun.  And perhaps doable if we turned off svg clipping, though it might require some cross browser work.  Possibly something to consider actually doing at some point.  Could make a fun sort of easter egg if nothing else.