GtkBox: don't propagate the expand child property

GtkBox: don't propagate the expand child property

The intention of the expand widget flags was to in fact propagate
legacy expand child properties as well. Due to implementation errors,
this was only happening in some cases, but not in others. To avoid
breaking old code assuming traditional expand flag behavior, this
commit removes all attempts to propagate GtkBox::expand.

This was discussed in
https://bugzilla.gnome.org/show_bug.cgi?id=698656 and
https://bugzilla.gnome.org/show_bug.cgi?id=628902

This patch was written by Tristan Van Berkom.