Switch to new WebIDE runtime API (bug 1069552)

This uses a new WebIDE API to cleanly add new runtimes, instead of hacking up random objects as before.

The API itself has not yet landed, so you'd need patches from bug 1069552 for now. I plan to uplift this work for Dev Edition too.

The changes in this add-on are currently **backwards incompatible**, so you would need an updated Firefox build after this. If this seems bad to someone, please speak up! I was hoping to just trash the old code, but perhaps people feel we need to keep it around.

I won't attempt to merge this change until the API lands in Nightly.