the 'BackgroundMusic' class defines the background music and the function used to control it, in the 'BattleshipApp' is an attribute of BackgrounMusic and a return function and in the Menu the Checkbox and the slider for the backgroundMusic and the update function were implemented
edited the property files to ensure, that in the menu the background music on/off and the background volume slider are named in eng and german, edited the childnotes for
the menu, so the checkbox and slider are visible, edited the enum Suonds, for another entry 'BACKGROUNDMUSIC'
