added background music and improved model code

reduced switch case for creating different ship models
created a new class for background music
added a button to toggle background music
added a slider so adjust volume
added new style for labeling the slider

Projekte/jme-common/src/main/resources/Interface/Lemur/pp-styles.groovy

@@ -55,6 +55,12 @@
     background.setColor(bgColor)
 }
 
+selector("slider_label", "pp"){
+    font = font("Interface/Fonts/Metropolis/Metropolis-Regular-32.fnt")
+    color = color(1,0.5,0,1)
+    textHAlignment = HAlignment.Center
+}
+
 def pressedCommand = new Command<Button>() {
     void execute(Button source) {
         if (source.isPressed())