SwingMessageWidget: allow labeled messages

Unlike other parameters, messages will only be labeled if a label was
actually specified. That is: a label will not be synthesized from the
parameter name when the label attribute is unset.

src/main/java/org/scijava/ui/swing/widget/SwingMessageWidget.java

@@ -60,7 +60,8 @@ public String getValue() {
 
 	@Override
 	public boolean isLabeled() {
-		return false;
+		final String l = get().getItem().getLabel();
+		return l == null || l.isEmpty();
 	}
 
 	@Override