Fix a crash in flatpak update notifier
Currently, the update notifier will crash due to a segmentation fault if a flatpak package has been updated.
The segmentation fault occurs because the installationChanged() callback expects the user data to have a type of FlatpakNotifier, but in reality it's FlatpakNotifier::Installation.
In order to fix the crash, this patch makes the installationChanged() callback cast user data to correct type.