More advanced tree view

[martinRenou]

As discussed with @trungleduc

Problem

It would be nice to implement a more advanced tree view making use of lab extensions. Even nicer would be to still support the static, template-based tree views custom templates are using nowadays (https://github.com/voila-dashboards/voila-material/blob/master/share/jupyter/voila/templates/material/tree.html for example).

Proposed Solution

By default, make use of a FileBrowser widget from JupyterLab. With a custom Drive that just looks into the tree view the server provides (very simple content manager that does not leak file contents).