Add sessions groups in the sessions tree

[kmagit]

When we have manies subjects in progress and we need to start new sessions for each to save tokens uses, it becomes hard to recognize which session is the last used and the attached subject.

It would be good to have a such of sessions groups so we know directly how to navigate in the tree and switch from one subject to another one :)

repo1
--group2
----session2
----session1
--group1
----session
repo2
--group2
----session2
----session1
--group1
----session1