Cleaner message handling

[hensm]

Message passing between the different destinations is fragile and difficult to understand.

Destinations:

• Background script (main)
• Shim content/page script
• Receiver selection popup
• Bridge app
• Options page (TODO Options page #5)
• Media control popup (TODO Media controls #10)

Need some way to handle this cleanly. Possibly a separate library or util class that allows for registering a destination with a callback for sending messages. Needs to provide a message handler function to be called from runtime.onMessage / port.onMessage.

Should also document existing messages and order in which they're sent during initialization.

[hensm]

27b5409 WIP