Add jsonrpc API

[tmpfs]

Should be available via a POST endpoint and over a websocket connection. The editor interface can keep a websocket open so requests/responses are as snappy as possible.

Customers that want to build their own interfaces can use whichever endpoint suits their use case best.

[tmpfs]

Preparation for this has been done with the creation of a JSON-RPC facade:

• https://github.com/tmpfs/json-rpc2/
• https://docs.rs/json-rpc2/