Will be emitted after the group is removed from the timeline through `ges_container_ungroup`.
Note that group will no longer contain its former children, these are held in children.
Note that if a group is emptied, then it will no longer belong to the timeline, but this signal will **not** be emitted in such a case.
| group |
The group that was removed from |
| children |
A list of Container-s that _were_ the children of the removed |