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.
Oh... ok. Got it, you are reusing Devices.jsm from scanner.js, but this is weird.
Wasn't the goal to replace Devices.jsm by RuntimeScanners?!
If we keep using it internally, we won't be able to remove it.
I would have listen for device-connected/device-disconnected directly from scanner.js.
(there should be a way to share Device() between main and scanner)
Or am I lost and we really do want to keep Devices.jsm ?!