Possibly fixes buggy co-deletion of SPointingConnections on deletion of
SStructure

- When deleting an SStructure with attached SPointingRelations, the last
SPointingRelation in the List (gotten from SDocumentGraph) was never
deleted.

- Possibly fixed by: Deleting all target- and sourceConnectionEditParts
in SStructureAdapter (multithreading-safe by using copy Lists) +
Refreshing *all* SStructureEditParts

- TODO: 

-- Check to refresh only those SStructureEditParts that are affected
(check whether Notification.SET is called for any of them in the
SStructureAdapter when modifying an SPointingRelation)

-- Check whether fix is really multithreading-safe (Could an SStructure
in the List over which refresh is iterating be modified after refresh?).

-- Check if refreshing can be made less expensive!