Allow closing a whole group#851

TL;DR: Add a CLOSE button at the right of the [Save Group as Collection] button.

Full Description:
The new feature to groups tabs with AI is very neat for organization. But sometimes I don’t want to create a collection… if I realize that a whole group is disposable, I just want to get rid of it (and all the tabs inside it).

Currently I have to do this on the browser only, but it would be much more efficient to do it inside of Toby too, so I can save what is useful and batch close what is not.