Allow empty tabs to act as roots of the tree

[noneuclideanmotion]

For example, I have a videos panel. It would be great if I could separate all programming videos with a pseudo-tab that I could rename as `programming* and then all my programming videos could be a child of the programming pseudo-tab (perhaps it could just be a new tab which you can rename)

[Kelendral-gh]

Select tab > Right-Click > Group