put the back and level-selection buttons into the "user-interface" so…

…und category, see phetsims/vegas#98

js/buttons/BackButton.js

@@ -49,7 +49,7 @@ class BackButton extends RectangularPushButton {
     // Create and add the default sound generator if none was provided.
     if ( !options.soundPlayer ) {
       const goBackSoundClip = new SoundClip( goBackSound, { initialOutputLevel: 0.35 } );
-      soundManager.addSoundGenerator( goBackSoundClip );
+      soundManager.addSoundGenerator( goBackSoundClip, { categoryName: 'user-interface' } );
       options.soundPlayer = goBackSoundClip;
     }