fixed some further bugs (checkbox and radiobutton behaviour)

...or/src/de/bmotionstudio/gef/editor/ButtonGroupHelper.java

...ionstudio/gef/editor/attribute/BAttributeButtonGroup.java

...rc/de/bmotionstudio/gef/editor/figure/CheckboxFigure.java

...de/bmotionstudio/gef/editor/figure/RadioButtonFigure.java

...itor/src/de/bmotionstudio/gef/editor/model/BCheckbox.java

...r/src/de/bmotionstudio/gef/editor/model/BRadioButton.java

.../src/de/bmotionstudio/gef/editor/model/Visualization.java

...de/bmotionstudio/gef/editor/part/AppAbstractEditPart.java

...r/src/de/bmotionstudio/gef/editor/part/BCheckboxPart.java

...rc/de/bmotionstudio/gef/editor/part/BRadioButtonPart.java