Switch to new WebIDE scanner / runtime API (bug 1069552)

Makes use of the new scanner / runtime API in WebIDE.

After this change, the add-on will still work with pre-API Firefox.

I've also added a Firefox for Android support, as it was helpful to build it to try out the runtime API.
I just thought "Firefox custom build" would have been more explicit than "Firefox custom", but that's not really important as people pushing a custom build knows that they are going off tracks.