Modify the IPackageMemberMgr.getNeighbourOf() method to know about merge
file groups.
Add a more meaningful error message if adding a file group into a merge
file group would cause a cycle.
Ensure that newly created merge file groups are initially placed to the
right of both of the input groups.
When a file group is added into a merge file group, record the new merge
group membership and bump pictograms to the right (if needed).
Draw connection arrows between sub file groups and merge file groups.