links in both directions between objects of the same class

[jvoigtlaender]

The file examples/XModeler-strange/testing_100.xml is deemed valid here, but XModeler rejects it.

The file is intended to correspond to this UML class + object diagram:

XModeler rejects it with the following message:

It then opens the diagram without one of the links encoded in the file:
and refuses (also in the GUI) to let the user add a y-link from c10 to c00.