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.
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.
