fix notebook editor focusing

[jonah-iden]

What it does

This fixes the focussing of notebook editors when switching tabs.
Previously there was the problem that it was neccessary to click the notebook editor once to set it as active

How to test

Best is probably through debugging.
Set breakpoints in applicaiton-shell.ts at onCurrentChangedand onActiveChanged and click the tab of a currently hidden notebook editor.
It should now focus correctly

Follow-ups

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

This looks reasonable, do you mind adding a tabIndex here as well?

theia/packages/notebook/src/browser/notebook-editor-widget.tsx

Line 274 in 80d68be

return <div className='theia-notebook-main-container'>