Implement widget API for signaling the widget to gracefully terminate

In theory, widgets could use iframe unload/beforeunload events for
cleanup, but in practice browsers have restrictions on what can be done
in those events which may not give sufficient time for clean
termination.

Signed-off-by: Pauli Virtanen <pav@iki.fi>

src/WidgetMessaging.js

@@ -87,6 +87,19 @@ export default class WidgetMessaging {
         });
     }
 
+    /**
+     * Tells the widget that it should terminate now.
+     * It is not necessarily called in all instances before the widget is removed,
+     * and the client may force termination with a timeout.
+     * @returns {Promise<*>} Resolves when widget has acknowledged the message.
+     */
+    terminate() {
+        return this.messageToWidget({
+            api: OUTBOUND_API_NAME,
+            action: KnownWidgetActions.Terminate,
+        });
+    }
+
     /**
      * Request a screenshot from a widget
      * @return {Promise} To be resolved with screenshot data when it has been generated