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.
This forget action removes the top bar item from the top bar order
representation, which is present in the settings, and then saves the top
bar order.
This action is intended for forgetting top bar items, which aren't
present in the top bar anymore, so that the user can clean up their
settings.
So if an item isn't in the top bar at the time of action activation,
activating the action removes it from the saved top bar order. If an
item is in the top bar at the time of action activation, the item
position just gets lost (since it gets automatically re-added to the top
bar order).
