Automatically delete the notification when action clicked

When the user clicks on the button that will trigger the action of
the notification, we delete the notification so it doesn't hang there
'forever'. Let's see how it goes, but I think it improves the usability.

CCMAIL:plasma-devel@kde.org

Signed-off-by: Artur Duque de Souza <asouza@kde.org>
Reviewed-by: Marco Martin <mart@kde.org>