Introduce a 'custom-item-activated' on the widget, which behaves
similairly to GtkEntryCompletion::action-activated, i.e. is emitted when
a custom item is chosen from the dropdown list.
Clients can use the name provided when adding the item as a detail for
the signal, to get notified when that specific item is activated, or use
the signal without details to get notifications for all custom items.