Added shout-button-message widget example.

[ericsnekbytes]

This example shows how to create a dual compatibility extension (for Lab and Notebook) by using optional features, and disabling them when they're not available.

[ericsnekbytes]

@fcollonval This is part of the same effort as the toparea text widget example if you can look this one over as well maybe?

[fcollonval]

Thanks @ericsnekbytes

You should improve a bit the logic to improve separation of concerns.

Could you please add an integration test and update the README to describe what the code is doing?

Two notebooks were committed by mistake.

[ericsnekbytes]

@fcollonval Can you re-review/approve? Thanks. EDIT: I need to add tests to this still actually.

[ericsnekbytes]

@fcollonval If you have time to review I'd like to get this approved, the Dual Compatibility document (see top post description) depends on this example so this should be merged first. My plan now is to try to get the document and examples merged first, then do a follow-up PR for the unit tests and repo-referencing for the code snippets.

[fcollonval]

Thanks @ericsnekbytes for the patience

The remaining CI failure is due to the need for the new doc in JupyterLab.