Add missing since tag to WidgetValueProperty.observe(Realm, S)

Change-Id: I70722d39d937bc2c0261bcc7743cba27a0b32727
diff --git a/bundles/org.eclipse.rap.jface.databinding/src/org/eclipse/jface/databinding/swt/WidgetValueProperty.java b/bundles/org.eclipse.rap.jface.databinding/src/org/eclipse/jface/databinding/swt/WidgetValueProperty.java
index 53cc494..f89dd6f 100644
--- a/bundles/org.eclipse.rap.jface.databinding/src/org/eclipse/jface/databinding/swt/WidgetValueProperty.java
+++ b/bundles/org.eclipse.rap.jface.databinding/src/org/eclipse/jface/databinding/swt/WidgetValueProperty.java
@@ -101,6 +101,9 @@
 		return new WidgetListener<>(this, listener, changeEvents, staleEvents);
 	}
 
+	/**
+	 * @since 3.14
+	 */
 	@Override
 	public ISWTObservableValue<T> observe(Realm realm, S source) {
 		return wrapObservable(super.observe(realm, source), source);