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)
suspect this is code copied from elsewhere, but i'd rather see blockBox declared at the top of the function instead of having the var in the if and in the else