Create Webapp

Webapp is a blank new app. I stopped short of putting any new logic in it so that it could be used as a snapshot as of this commit for anyone else who needs to create new app. 2 levels exist by default: simple and full_sandbox.

We are planning to use it for CSP to expose new object model. Unlike the other apps that render to SVG or Canvas, Webapp just has an HTML DIV in the visualization area (play space). The forthcoming object model will allow the creation of other HTML elements in that area.

Also, made a minor change to the visualization.responsive styling so that all child elements of the visualization element will be scaled appropriately (previously, it was just svg and canvas child elements).