SelectedProductEvent: now working as a real event
SelectedITemEvent: now working as a real event
ProductSelectionListener: better interface to fit the event system
ItemSelectionListener: better interface to fit the event system
CategoryItemList: changed to fit the ItemSelectionListener interface
CategoryView: now works as a middle dispatcher
CollectionView: code fixed to fit ProductSelectionListener interface
DiagramPanel: code fixed to fit ItemSelectionListener interface
MainWindow: implements the event listeners correctly now