- added a class BackgroundMusic: is an AbstractAppState and GameEventListener that handles the backgroundmusic
- attached the BackgroundMusic to the stateManager in the BattleshipApp
- added to the Menu a CheckBox and Slider to manipulate the volume of the backgroundmusic
- added four different music files (for different states of the game)
- edited the WaitState and BattleState to play different music files when chaing to that state
- added to ClientGameLogic a new method playMusic(Music) to play the right music (depends on the current state)
- added a new method receivedEvent(MusicEvent) to handle the music events
- added a new enum Music, that represents different types of music
- added a new record MusicEvent(Music), that decides which music shall play
