Allow communication between outputframes

[blois]

Trusted outputframes should be able to send and receive messages with other trusted outputframes.

This is to allow widgets in one outputframe to affect the visualization in another one.

[blois]

The code would look something like:

// A callback invoked from other cells.
google.colab.registerListener('cell1', (id, data) => {
  // do something with data.
});

Then that method in one output would be able to be invoked from another cell's output:

google.colab.sendMessageToListeners('cell1', { text: 'hello world!' });

[blois]

A better approach for this is to use the browser's BroadcastChannel to communicate between frames.

An example can be found at https://colab.research.google.com/notebooks/snippets/advanced_outputs.ipynb#scrollTo=8VlhVnsL-eKX