Add structural link to editor

[vainolo]

Add structural link to editor

[vainolo]

• Created OPMStructuralLinkAggregatorEditPart extending OPMNodeEditPart, adding it to the factory and testing using dummy class created in the OPD, and a rectangle figure.

[vainolo]

• Added default value for OPMNode constraint in model. So it is always initialized.
• Created OPMStructuralLinkAggregatorFigure
• Created IsoscelesTriangle figure for use in OPMStructuraLinkAggregator.
• Created OPMNodeFigure interface, superinterface of OPMThingFigure interface. Defines that a node figure must have
  source and target anchors. OPMThingFigure now only defines existence of a name label.
• Pulled up get*Anchor in class OPMThingEditPart to OPMNodeEditPart, which now implements NodeEditPart (which was implemented before by OPMThingEditPart). Changed casting of figure from OPMThingFigure to OPMNodeFigure.
• Created OPMStructuralLinkAggregatorFigure.
• Modified OPMObjectProcessDiagramXYLayoutPolicy so that OPMStructuralLinkAggregator cannot be resized, only moved.

[vainolo]

• Created factories for exhibition, aggregation and generalization instances.
• Updated OPMNodeGraphicalNodeEditPolicy so that connection cannot start or end at aggregators.
• Added structural link creation tools.
• Modified OPMNodeGraphicalNodeEditPolicy to create structural link aggregators, in a default location. Working.

[vainolo]

• Fixed aggregator figure to handle Exhibition links.
• Added access methods in model to get outgoing and incoming structural links (currently implemented in a very naive way).

[vainolo]

• Create structural link in OPMNodeGraphicalNodeEditPolicy now handles creation of structural links correctly.
• Node removal now handles removal of nodes connected to structural links: if there are outgoing structrual links it removes the aggregators (which in turn removes all the links connected to the aggregators). if the node is a target of a structural link that is the only link from an aggregator the aggregator is also removed.