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.
Scroll the content of the preferences window on Drag-and-Drop, when the
mouse is in the upper or lower part of the content of the preferences
window.
This helps a user, who has e.g. a lot of items, but the preferences
window with a low height, to easily move any item to any position.
