Align "Add Folder" visibility with VSCode

[colin-grant-work]

What it does

This PR aligns the visibility of the context menu command to add a folder to a workspace with the corresponding command in VSCode.

How to test

On master, the command is visible if:

• You right-click in empty space in the Navigator widget in an unsaved single-root workspace.
• You right-click in on an existing root in the Navigator widget in a saved or multi-root workspace.

With these changes the command is visible if:

• You right-click in empty space in the navigator widget in any workspace.

On master, there may not be any empty space in the Navigator widget. This PR adds a margin to a React virtualized scroll container to ensure that there is always some empty space at the bottom of the list, as there is in VSCode.

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

[vince-fugnitto]

The changes work well for me 👍
I confirmed that:

• the command is available from the more toolbar item
• the command is available from the empty space in the explorer
• the command from the command palette works well, and is properly enabled

[vince-fugnitto]

I confirmed that the extra space works well, do we want to add it to all trees or just the explorer?

[colin-grant-work]

I think we should have it for all trees, since all trees have (by default, anyway) listeners for clicks targeting their nodes but no particular tree node, and that is only guaranteed to work if we ensure that all trees have some empty space. But I don't feel very strongly, since the Navigator is the only place I'm certain it matters.

[vince-fugnitto]

I've only seen it being used in vscode for the explorer so I asked the question, but it would probably benefit all trees so we can keep it generic like you have it.

[vince-fugnitto]

Confirming approval 👍