Add possiblity to create internal nodes

[vainolo]

Add possiblity to create internal nodes

[vainolo]

• Added a new interface to the model: OPMContainter, which is a container of nodes. Refactored OPMNode and OPMObjectProcessDiagram so that they both implement this interfaces.
• OPMLinks are still contained by the OPM, and OPMNodes are contained by their container.
• A method OPMNode.getOpd was implemented to be used when creating links. The implementation travels the OPMContainer hierarchy until if finds the OPD.

[vainolo]

• Changed OPMObjectProcessDiagramXYLayoutPolicy to OPMContainerXYLayoutPolicy and made it avaliable to all containers (OPD, Object, Process).
• OPMThingFigure now includes a contentPane where the model child parts are added (and not directly to the matching figure). This is how it is implemented in the logic example. Had to override the findFigureAt function of the figure so that it finds the child figures in the contentPande when the main figure gets a click.
• There is a strange problem while moving parent figures... Will need some debuging in the future.