Add no-arg constructor for CopyableLabel

This constructor makes it easy to include CopyableLabel in UiBinder
XML code and populate the strings at runtime.

Change-Id: Ia2c839bd9e4ef004e2f7ee03050391aa0e4b245e

gerrit-gwtexpui/src/main/java/com/google/gwtexpui/clippy/client/CopyableLabel.java

@@ -71,6 +71,10 @@ public class CopyableLabel extends Composite implements HasText {
   private TextBox textBox;
   private Element swf;
 
+  public CopyableLabel() {
+    this("");
+  }
+
   /**
    * Create a new label
    *