Hide empty plugin view containers from user

[msujew]

What it does

For some reason, the jupyter notebook extension registers an empty view container. See here. Theia shows the empty container in the Views menu and even allows the user to open it (even though it is completely empty):

However, this doesn't matter in vscode, as empty view containers (i.e. view containers which never have a view registered to them) are just not visible to the user in any way.

This change aligns our UX to vscode and only registers the appropriate commands/menu entries once a view has been added to the view container.

How to test

1. Install the Jupyter plugin
2. Only one Jupyter entry should be visible in the Views menu
3. Only one View: Toggle Jupyter command should be visible
4. All other plugin view containers should work as expected

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

[jonah-iden]

works as described and looks good to me