Sidebar - Mail like sidebar with labels and trees

[frankbolviken]

So the last day I have been struggling to achieve the same functionality as the mail app:

Basically I would like to add these grey label texts above different sections of the sidebar. Is it possible to achieve something like this with the current implementation?
Ive tried different approches, like adding a Column in the sidebar builder etc but getting errors like invalid vertical space ++.

[Adrian-Samoticha]

Related: #292

[Adrian-Samoticha]

#543 resolves this.