It should be possible to `dispose()` the `Application` object

[krassowski]

Problem

Application once started cannot be disposed; this prevents its use in many larger applications. It is not even possible to add it in subclass because for example CommandRegistry has no dispose() and even if we subllass that too then we cannot swap it before constructor assigns the default CommandRegistry to context menu instance.

Proposed Solution

Make Application implement IDisposable.

Additional context

None

[jasongrout]

While I think having a dispose method for an application seems like good practice, in the Jupyter Frontends call today @bollwyvl also brought up that if the goal is guaranteed clean isolation and cleanup of a JupyterLab instance, the right solution is probably putting the application instance in an iframe, and then getting rid of the iframe to clean everything up.