Default actions are the actions triggered when the notification is simply
clicked, without adding any "action buttons". Until now, with
KNotifications we could only make notifications that go away when clicked,
which might not be what the user expects (see notifications on smartphones
or other desktop environments).
Since KNotifications uses an int->name mapping (instead of string->name),
we need to manually convert between "default" and "0". Interestingly
enough, there was already code to handle the action "0" and emit
activated(), which is documented as "Emitted only when the default
activation has occurred" but in practice was never actually used.