Add the `PrefsBoxOrderListBox`es to the `PrefsPage` via the
`PrefsPage`es UI file.
Also let the `PrefsBoxOrderListBox`es initialize themselves based on the
given box order.
Move the logic for saving the box order represented by a
`PrefsBoxOrderListBox` (and its `PrefsBoxOrderItemRows`) to the
`PrefsBoxOrderListBox` class.
This makes the code cleaner and allows for easy reuse of the logic in
the future.
Create a custom class (`PrefsBoxOrderListBox`) for GtkListBoxes, which
are used in the preferences window for holding box order settings.
Having a custom class for these GtkListBoxes is useful, since it can
hold logic related to box order settings. Future commits will bring more
of this logic into this class.