Util: New utility class for UI message queues, for use by i2psnark and i2ptunnel

i2psnark: Use new utility, prevent message loss on clear
i2ptunnel:
- Don't lose messages on refresh (ticket #2107)
- New clear messages button
- Hide message box if none
- javadoc clarifications

router/java/src/net/i2p/router/RouterVersion.java

@@ -18,7 +18,7 @@ public class RouterVersion {
     /** deprecated */
     public final static String ID = "Monotone";
     public final static String VERSION = CoreVersion.VERSION;
-    public final static long BUILD = 11;
+    public final static long BUILD = 12;
 
     /** for example "-test" */
     public final static String EXTRA = "";