Fix for issue #577 StringBlock cannot be cloned with the cloner because

it is package private and the cloner cannot instantiate one directly.
Since it is extremely unlikely (read: impossible) that there would ever 
be shared StringBlock references between BitmapText objects then it is safe 
to just clone it directly.
It is important to note that BitmapText never really did support clone
before and only pretended to... so this wasn't really a regression.

jme3-core/src/main/java/com/jme3/font/BitmapText.java

@@ -95,7 +95,12 @@ public class BitmapText extends Node {
         for( int i = 0; i < textPages.length; i++ ) {
             textPages[i] = cloner.clone(textPages[i]);
         }
-        this.block = cloner.clone(block);
+        
+        // Cannot use the cloner to clone the StringBlock because it
+        // is package private... so we'll forgo the (probably unnecessary)
+        // reference fixup in this case and just clone it directly.
+        //this.block = cloner.clone(block);
+        this.block = block != null ? block.clone() : null;
 
         // Change in behavior: The 'letters' field was not cloned or recreated
         // before.  I'm not sure how this worked and suspect BitmapText was just