Correctly handle conversation windows being opened at the same time as receiving an incoming call

This fixes the issues in this [trello card](https://trello.com/c/kk7HhDzd).

The basic issue is that if an incoming call is received just after the user opens the conversation window, but just before it is fully loaded, then the port isn't known and the message gets lost.

This patch adjusts the ice candidate queuing system to work for the incoming call messages as well, so that any messages to the window are queued until tthe port is open and ready.