[Notifications] Never manually hide() the NotificationPopup

Otherwise the positioner will get completely confused.
When a notification is closed, the popup will already be closed in response to sourceRemoved.
Also, when triggering an action, call closePopup instead of hiding the popup.

(cherry picked from commit b97fdfa293dd4de59046c19f08aa6a3b940a790c)

Differential Revision: https://phabricator.kde.org/D4632