WidgetManger does not support mutliple kernels

[miduncan]

The current implementation of WidgetManager only support single kernel notebooks, so if loaded in a Notebook with multiple kernels it will have unusual behavior

[miduncan]

There is a fix for this in the branch miduncan/multiple-kernels, but I didn't have time to write a test for it. If this is a blocking issue for someone, feel free to branch and complete

[captainsafia]

@miduncan Thanks for submitting the PR.

In general, following the merging of the PR to remove currentKernelRef and adding improved support for kernel-content relationships, we need a multi-notebook test canvas.

We'll probably be able to do this with the sidebar component in the upcoming UI overall.