Breaking: Make preferences usable again and support Gnome Shell 42

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.

src/prefs-box-order-list-box.ui

@@ -2,7 +2,7 @@
 <!--
   This file is part of Top-Bar-Organizer (a Gnome Shell Extension for
   organizing your Gnome Shell top bar).
-  Copyright (C) 2021 Julian Schacher
+  Copyright (C) 2021-2022 Julian Schacher
 
   Top-Bar-Organizer is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
@@ -21,5 +21,8 @@
     <template class="PrefsBoxOrderListBox" parent="GtkListBox">
         <property name="selection-mode">none</property>
         <property name="show-separators">True</property>
+        <style>
+            <class name="boxed-list"/>
+        </style>
     </template>
 </interface>