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.
Introduce box orders, which allow this extension to save the order top
bar items should be in.
On extension enable, save new top bar items to those box orders.
I'm using version 1 for the first development phase (like what major
version zero (0.y.z) is in Semantic Versioning 2.0.0), so that the first
production version then gets version 2.