Changes for https://github.com/the-djmaze/snappymail/issues/37

Testing van be done at https://snappymail.eu/demo/

dev/serviceworker.js

@@ -0,0 +1,15 @@
+'use strict';
+
+self.addEventListener('message', event => self.client = event.source);
+
+const fn = event => {
+	self.client.postMessage(
+		JSON.stringify({
+			data: event.notification.data,
+			action: event.type
+		})
+	);
+};
+
+self.onnotificationclose = fn;
+self.onnotificationclick = fn;