restore notebook toolbar under jlab4 - fix for issue #1107

[parmentelat]

a fix for solving issue #1107

here's what both widgets (Notebook and Jupytext Notebook) look like

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as can be seen on the screenshot there remains issues related to extensions
here I have installed jupyterlab-courselevels that adds some buttons in the toolbar; and what happens is that these buttons show up in the pure Notebook (ipynb) widget but not in the Jupytext Notebook widget

IMO this PR is still very welcome because

• the situation is far better than it was before this work
• I suspect that getting this to work perfectly is something very tricky
• especially considering that we build in a jlab3 environment, where some of the symbols that are, I think, needed to solve this properly, are not defined to typescript

So @mwouts I would be grateful if you could publish this under pypi - it is already versioned as 1.15.0.dev2