Work in progress prototype "remote inspectTarget" for devtools addons 

Proof of concept of a RDP actor which exposes instrumentation, message exchange and javascript evaluation features needed by DevTools add-ons to support remote targets:
- bugzilla issue [980481](https://bugzilla.mozilla.org/show_bug.cgi?id=980481) 

Notes about the current prototype:
- it needs to be completed to support more than 1 "addon devtool panel" client per actor instance (e.g. maintaining 1 worker x add-on devtool panel defined or 1 RDP connection x add-on devtool panel)
- ContentObserver is included in the WebGLActor as a private helper and temporarily copied here