Make the preferences usable again (and look good) by using an
`AdwPreferencesPage` as well as various other libadwaita features (like
`AdwPreferencesGroup`, style classes, ...).
Also get rid of some code, which now doesn't seem to be necessary
anymore (making sure no horizontal scrollbar shows and setting a default
size for the preferences window).
This change is marked as "Breaking", since I assume `AdwPreferencesPage`
doesn't work in Gnome Shell 40 and because I changed the supported Gnome
Shell version to 42 accordingly.
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.
