Webapp changes

minor changes from 10 days ago, waited to merge until after HOC:

* Remove comments at the top of the JavaScript code telling the student which functions are available (better served in real UX elements, not needed in block mode either)
* Minor gutter fix in droplet - corresponds to https://github.com/dabbler0/droplet/pull/26
* remove "contenteditable spellcheck=false" from codeTextbox - this was a holdover from before droplet. It caused unexpected problems.
* move droplet styles into common style file, add more droplet palette styles to match what pencilcode is doing
* changed marshalNativeToInterpreter() to default to infinite depth while marshaling. Override to maxDepth=4 only in the webapp nativeGetCallback where we've had stack depth problems with DOM objects in events.