Fix the config box treeview width in GTK3.

Fix the config box treeview width in GTK3.

My trickery in GTK2 to start with some branches of the tree collapsed
but give the widget all the width it will need when they open later
was not working in GTK3, for the same reason I've needed several other
fixes recently: just after creation, GTK3 widgets report their
preferred size as zero.

Fixed by doing basically the same trick I was doing in GTK2, but
deferring it until the "map" event happens later on.