sce: child states can be hidden

...-plugins/StatechartEditorPlugin/StateEditorController.cpp

...ins/StatechartEditorPlugin/StatechartEditorController.cpp

.../StatechartEditorPlugin/view/StatechartEditorMainWindow.h

...StatechartEditorPlugin/view/StatechartEditorMainWindow.ui

...marXGui/gui-plugins/StatechartViewerPlugin/StateScene.cpp

...ArmarXGui/gui-plugins/StatechartViewerPlugin/StateScene.h

.../gui-plugins/StatechartViewerPlugin/view/MorphingItem.cpp

...ui/gui-plugins/StatechartViewerPlugin/view/MorphingItem.h

...Gui/gui-plugins/StatechartViewerPlugin/view/StateItem.cpp

...rXGui/gui-plugins/StatechartViewerPlugin/view/StateItem.h

...ui-plugins/StatechartViewerPlugin/view/StatechartView.cpp

.../gui-plugins/StatechartViewerPlugin/view/StatechartView.h