fix: don't do self-destroy in LibnotifyNotification::Dismiss()

Callers of Notification::Dismiss() assume that the notification
instance is not deleted after the call, but this was not the case
for LibnotifyNotification:
- Destroy() would get `this` deleted.
- notify_notification_close() in portal environment triggers
LibnotifyNotification::OnNotificationClosed(), and finally calls
Destroy()

This patch removes all Destroy() in Dismiss(), and adds a boolean
to tell whether notify_notification_close() is running, to avoid crash
under portal environment.

Fixes #40461.

Co-authored-by: taoky <me@taoky.moe>