Add blockly modal function editor

Goals:
1. Hide function definition when not being edited
2. Improve discoverability of parameters
3. Improve discoverability of creation
4. Improve discoverability of edit

This change builds on the following WIP review:
[#244 [WIP for review] Support multiple top-level SVG Workspaces and Toolboxes](https://github.com/code-dot-org/code-dot-org/pull/244)

![screen shot 2014-11-03 at 4 10 51 pm](https://cloud.githubusercontent.com/assets/413693/4892812/70b6b96e-63b7-11e4-82ed-5790da63450a.png)
I think I'd prefer relying on the `goog.array` versions of iterators so we get polyfills by default, though it doesn't look like we support and browsers for which `forEach` would be an issue.

I vaguely recalled us coming across a regular `forEach` issue with a particular browser in the past? Or was that `for in`?