navigator: handle empty multi-root workspace

[paul-marechal]

When a multi-root workspace is opened, the navigator will try to draw
the file tree because model.root is a WorkspaceRootNode with no
children.

This commit handles this case by adding a bit of ui to help the user add
folders to his workspace, instead of displaying a blank view.

Signed-off-by: Paul Maréchal paul.marechal@ericsson.com

How to test

• Open multi-root workspace.
• Remove all folders from it.
• A button to add folders should be displayed.

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

[colin-grant-work]

When I tried running this, I still got to an empty file tree:

My steps were:

1. Open a folder as a workspace.
2. Add another folder to the workspace.
3. Save the workspace as a workspace file.
4. Remove both folders from the workspace.

[paul-marechal]

When I tried running this, I still got to an empty file tree:

There seems to be something funny happening here. I'll look into it, thanks!

[paul-marechal]

@colin-grant-work @vince-fugnitto ok I added a bit of documentation because I got confused with how the tree model worked for single vs multi root workspaces. The logic should be fixed.

[colin-grant-work]

The functionality is working for me, and the code looks good. 👍 This message and behavior is very similar to the 'no workspace' message, so a case could be made for combining the two renderers, but I think it's likely better to keep them separate.

[DucNgn]

I tested and confirm the functionality works as described. The code also looks good to me.
Just a small nitpick is I dont know if we should avoid contractions in theia UI?
Your workspace doesn't contain any folders.

[vince-fugnitto]

The changes work well 👍
Thank you for also including documentation in the updates.