Fully hierarchical organization (tree outline style)

[cxw42]

Windows as children of windows, &c. Requested by Blank Makkara via user review 2017/09/07.

Edit Also requested by Terrn Lister via support request 2018/11/17.

[philiprhoades]

This would indeed be an enhancement but actually, since shifting to TF I have found that being forced into a "flatter" (narrower?) structure has had some advantages in terms of navigation . . #35 remains a higher priority for me . .

[cxw42]

Now that Chrome has tab groups, #246 might be an easier first step. I would still like fully-hierarchical eventually.