OPEN - task 79: Improve Object, Process and State figures 
https://github.com/vainolo/Object-Process-Programming/issues/issue/79

* Renamed OPMGenericLabelFigure to OPMLabelFigure
* Renamed LabelEditPart to OPMLabelEditPart
* Fixed references to OPMLabelFigure in OPMLabelEditPart and
OPMThingEditPart to use the OPMNamedElementFigure interface.