Toggle WebRTC wot GarryW

A toolbar button to toggle WebRTC

Disabling it protects local IP from leaking.

Users who want to protect local IP can set `media.peerconnection.enabled` in `about:config`, but that breaks some web feature. So I made this button to toggle it.

Support setting keyboard shortcut.

## Other addons

https://garywill.github.io/#Browser-Addons