Pasting partial sequence diagrams leads to crashes

[prmr]

Steps to reproduce:

1. Create a sequence diagram with at least one call edge between two implicit parameter nodes
2. Select only the call edges and call nodes.
3. Copy the selection.
4. Open a new sequence diagram graph
5. Paste the selection

The application will hang.

The problem generalizes to pasting children nodes without their parent.

[prmr]

Problem 1: The graph area grow/clip feature relied on a null value.

[prmr]

Problem 2: The paste feature pasted children nodes with a reference to a parent node, when the parent node was not in the clip board. If children nodes are pasted without their parent, and this is a legal subgraph, the links to the (obsolete) parents should be clipped so the nodes can be made root nodes of the destination graph.