messageTray: Do not show low priority notifications

Low priority notifications are meant for information that doesn't require
immediate attention, so it makes sense to not show a banner for them;
they'll still appear in the notification list in the calendar.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4265

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1893>

js/ui/messageTray.js

@@ -771,6 +771,9 @@ var Source = GObject.registerClass({
         notification.acknowledged = false;
         this.pushNotification(notification);
 
+        if (notification.urgency === Urgency.LOW)
+            return;
+
         if (this.policy.showBanners || notification.urgency == Urgency.CRITICAL)
             this.emit('notification-show', notification);
     }