GUI improvment: Spinner, ComboBox, PreWindowScreen
The gui components - especially Spinner and ComboBox - are designed better and they're looking quite nice right now. I also removed a bug, in which the selected Index has been set correctly, however the listeners were tiggered after ComboBox/List so there was no change anyway. I still got a problem in PreWindowScreen: Somehow he doesn't want to change back to "Computerstärke" in selectorComboBox.