Let `PrefsBoxOrderItemRow` handle the association of itself with an item
itself.
Doing so makes the code cleaner, since the association isn't done
externally anymore.
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.
Previously it could happen that the preferences window got larger than
the height of the screen (e.g. when having a bunch of items and a low
vertical screen resolution).
Fix this, by making the preferences window content scrollable.
Also introduce a nice default window size.
