Organize previews in sections

[emaklita]

Right now the tree of previews can be customized in a simple way. This is a great feature 👏

In our team, as the list of previews is growing, we are looking for a way to organize them into sections. Where each section has its own tree of previews.

This would give a good overview of the content of each section when the trees are collapsed, and also reduce the number of nesting levels in our previews (less clicks to expand the subfolders in the nav).

Here is how the UI could look like:

Right now without sections

With sections

[emaklita]

This issue is also related to the organization of previews in the sidebar #125