Add support for webapps actor

People keep asking for being able to push apps to b2g desktop/device from the command line.
firefox-client looks like the best implementation of devtools protocol client and I would like to promote it as the best way to write a command line tool for b2g.

Here is a pretty significant patch, that allows accessing the webapps global actor (bonus: it also exposes the root actor) and implement various helper to ease using all webapps actor feature.

Thanks you so much @harthur for providing this library, it was really easy to tweak it to support webapps actor!!

Here is a hardcoded test app script that allows testing all these methods: https://pastebin.mozilla.org/5094843

@RudyLu, you pinged me sooner this morning. I did that by thinking about your usecase. Hopefully you can use this for your needs! (See the pastebin)