Global output area for the notebook

This is something that has come up a few times in the past in discussions on various topics such as connecting multiple clients to a kernel and interactive widgets.

There should be a global output for the notebook, not only in the UI but also in the document format. For the following reasons:

1. There are multiple cases where `stdout` / `stderr` messages are not uniquely bound to a cell of the notebook and are therefore lost to the user.

- an output message broadcast from another client connected to a kernel (e.g. a console)...
- an stderr / stdout message that comes from events that originated from a widget model message (and not a view) which are not bound to a cell.

2. This information should probably part of the notebook format. This kind of notebook deserves to be saved as much as the cell output area.

If we do so, information such as the widget manager state could become global output instead of notebook-level metadata. Indeed such an output would be a more natural location for transient information than `metadata`, which conveys the idea of more static information.

Not all front-ends need to implement a UI for that notebook-level output area, but I imagine that it could be displayed optionally, or simply accessible through convenience js fonctions for people who work in the front-end.