gtk-builder-tool simplify --3to4 now produces a
<requires lib="gtk" version="4.0"/>.
Adapt for that, and also add some tests that check that we
actually verify the requires, and accept 3.99 and 4.0, but
not 5.x.
Rewrite expand/fill properties on GtkBox to
hexpand/halign/vexpand/valign on the child widget.
Rewrite GtkVBox and GtkHBox to GtkBox, setting the orientation
property.
Added a test for boxes.