preferences-group: Add GtkListBoxRow children to the listbox
While it is always possible to prevent GtkListBoxRow from being added to the listbox by wrapping them in anything but a HdyPreferencesRow, and while it is possible to add any widget to the listbox by wrapping them in a HdyPreferencesRow, wrapping a GtkListBoxRow in another is unintuitive and feels incorrect. Hence, this directly adds them to the listbox.
This is needed to use HdyPreferencesGroup
in Settings.