added MainVolume slider in the menu

added the logic so that the volume of music or sound is multiplied by the main volume
created the class MainVolume to handle the logic of the main volume
adjusted methods in GameSounds and BackgroundMusic to integrate main volume

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

@@ -36,6 +36,7 @@
 selector("label", "pp") {
     insets = new Insets3f(2, 2, 2, 2)
     color = buttonEnabledColor
+    textHAlignment = HAlignment.Center
 }
 
 selector("header", "pp") {