Draft: Attempt to migrate UI files with 'gtk4-builder-tool simplify --3to4'

For layouts that used to use GtkButtonBox with packing, the packing was removed because a matching layout was not immediately apparent.

The UIs need to all be checked for correctness.

This is an attempt to start fixing #10 (closed), and gnome-control-center issue 1688 (and matching Fedora downstream issues[2][3]).