[Bug] Disbanding a group cannot be undone

[letsfindaway]

Steps to reproduce:

• Create a group of items.
• Disband the group by pressing the button in the group delegate frame. The group is removed and the individual items are selected.
• Then press "Undo". The group is not re-created.

Short analysis

Destroying a group using the delegate button calls destroy() on the container item. This first removes all the items from the group, making them individual items again. Then it calls remove() at the UBItem, wh9ich just forwards it to remove of UBGraphicsItemDelegate. This removes the container item from the scene and creates an undo command, which adds the group item again, but does not care about the items previously in the group.