Fix some miscellaneous weirdness around push notifications

For some reason, I *stopped* the timer if we get a push message - which
makes *no* sense. It meant that NeoChat kept running in the background
for basically no reason.

We can also make the NotificationsManager::postPushNotification function
static and avoid creating some singletons.

(cherry picked from commit 94ea1305b2)

src/app/controller.cpp

@@ -318,8 +318,7 @@ void Controller::listenForNotifications()
     connect(timer, &QTimer::timeout, qGuiApp, &QGuiApplication::quit);
 
     connect(connector, &KUnifiedPush::Connector::messageReceived, [timer](const QByteArray &data) {
-        instance().m_notificationsManager.postPushNotification(data);
-        timer->stop();
+        NotificationsManager::postPushNotification(data);
     });
 
     // Wait five seconds to see if we received any messages or this happened to be an erroneous activation.