Exception on un-collapsing a group

[Danil Sokolov]

To reproduce:
 1) Create an STG with two connected transitions
 2) Select everything and grate a group
 3) Collapse the group
 4) Move the group to a new position
 5) Un-collapse the group.

Observed behaviour:
 - Stack overflow exception is thrown.

Expected behaviour:
 - The group should be un-collapsed with no exceptions.

Note that the same stack overflow issue is observed both for Polyline and Bezier connections.

This is due to an infinite recursion on updating the bounding-box of the connection.