NotificationManager rework

Rework notifications manager to no longer be a singleton, but a component of controller.

The dependency on it for neochat room and connection is also removed.

src/neochatconnection.h

@@ -209,6 +209,11 @@ Q_SIGNALS:
      */
     void errorOccured(const QString &error);
 
+    /**
+     * @brief Request a notification be shown for an invite to this room.
+     */
+    void showInviteNotification(NeoChatRoom *room);
+
 private:
     bool m_isOnline = true;
     void setIsOnline(bool isOnline);