DRAFT: Single widget manager per kernel

[fleming79]

Replaces https://github.com/jupyter-widgets/ipywidgets/pull/3920.

Features:

• Widgets should be able render anywhere the WidgetRenderer can be accessed (anywhere the global rendermime or clones work).
• Correctly indicates when comms is disconnected.
• Restore widget connection when session context.
• easier to manage and find widgets - the KernelWidgetManager.

Significant changes

• The KernelWidgetManager is a single instance per kernel by the kernel ID.
• The KernelWidgetManager stores and creates widgets.
• The default rendermime will actively search for the [Kernel]WidgetManager that owns the widget model.
• WidgetManager (not to be confused with KenelWidgetManager) has been re-purposed to track the renderers belonging to a session context and set the KernelWidgetManager when the kernel is loaded. WidgetManager informs the renderer that the [Kernel]WidgetManager is pending.
• Re-rendering on demand.

Notebooks and Consoles can share widgets.

When the notebook closes (its kernel connection has the comms), the connection is broken.

Opening a new kernel connection (in this case a console) restores the connection.

Closing the console that has the kernel comms connection again breaks the connection.

Shutting down the kernel breaks the connection.

Loading a new notebook (that was tracking the widget state) with a new kernel, doesn't have widgets in the kernel, so the widgets appear with broken links.

Connecting a notebook to another kernel means the connection to the widgets in the original connection remain unbroken when the notebook is closed.

[github-actions[bot]]

:point_left: Launch a binder notebook on branch fleming79/ipywidgets/per-kernel-widget-manager