Integrate Tor Launcher into tor-browser

We need to move away from our XPCOM extensions, Tor Launcher being one of them. As for Tor Browser it might be hard/impossible, if we tried to reimplement everything Tor Launcher does with the WebExtensions API. Instead we plan to integrate it tighter into the browser itself making use of its capabilities.

This ticket is the parent ticket for this plan.

We need probably a proposal making sure we have the plan right before going to implement it.