Allow connecting from other extensions

[Powersource]

We are considering using Scatter instead of MetaMask but at the moment we can't since afaict only websites can use Scatter, not web extensions. MetaMask has support for this even if it's pretty hacky so far.

[nsjames]

This will not be supported from within this "Scatter Classic" extension, but can be supported through the Scatter Desktop extension.

https://github.com/GetScatter/ScatterDesktop

There is no documentation for this yet, but working example code is used in the new light extension that connects with the desktop app:
https://github.com/GetScatter/ScatterWebConnector/blob/master/src/services/SocketService.js