Allow default action without label
I don't know whether it's too late, I hope it's not, I propose defaultAction/setDefaultAction to become std::optional<QString> in KF6 (or a check for isNull, or some other way) so it's possible to set a default action without label and to have an action on notification click but have no button associated with it. This would help xdg-desktop-portal-kde a lot as portal API has no label for default action and it specifies internal ID as label: https://invent.kde.org/plasma/xdg-desktop-portal-kde/-/blob/master/src/notification.cpp#L72-75 which results in one can see buttons labeled as e.g. "app.notification-open".
Edited by Ilya Fedin