Allow callers of AddMemberBox to control the button text

This way the caller can customize the action text to be specific
to what will be performed when the user presses the button.

Signed-off-by: Shawn O. Pearce <sop@google.com>

src/main/java/com/google/gerrit/client/ui/AddMemberBox.java

@@ -50,7 +50,11 @@ public class AddMemberBox extends Composite {
     
     initWidget(addPanel);
   }
-  
+
+  public void setAddButtonText(final String text) {
+    addMember.setText(text);
+  }
+
   public void addClickListener(ClickListener listener) {
     addMember.addClickListener(listener);
   }