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)
Agreed worth simplifying this pattern at some point, I'm hesitant to do that now just due to the string <> boolean <> XML relationship, the parallel usages above and not knowing what the callers are sending in at the moment. (`!!("false")` evaluates to `true` so that particular simplification would scare me a bit as well)

Think it's worth tracking separately?