Notify parent window about room state

Notify the window.top with postMessage about changes in the room state.

When using screego in on a dashboard TV, some may want to display some other stuff when there isn't a shared screen active (discussed in https://github.com/screego/server/discussions/239). Screego could notify the parent window about changes in the room state (like someone started / stopped streaming).

This way, screego could be embedded as iframe, and the parent window decides what to display.