Visually show the tab groups in sessions

[susancolvine]

I know tab groups are already supported, but it would be a huge improvement if we could actually see them within our saved sessions. Specifically, it would be great to have a visual layout that shows tab groups, with their names along with all the tabs inside each one. This change would definitely make organizing sessions a lot more intuitive and easier. At the moment, the tab group feature feels a bit unfinished. Being able to see those groupings in our sessions would make the experience feel really complete.