DRAFT: Single widget manager per kernel

[fleming79]

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

Features:

• Widgets can render anywhere the WidgetRenderer can be accessed (anywhere the global rendermime or clones work).
• Correctly indicates when comms is disconnected.
• Re-renders widgets when comms is restored.
• 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 is closed ipwidgets takes over comms and re-renders the widget.

Shutting down the kernel breaks the connection (note the red paper clips)

Re-opening the notebook now gives a message

Model not found for this kernel: AppLayout(children=(HTML(value='<h1>Fictional World Weather</h1>', layout=Layout(grid_area='header', height='a…

[github-actions[bot]]

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