Because "size-request" signal can be handled by signal connections as well
as by third party derived classes it is necessary to always fire the
"size-request" signal for every recalculation of the widget requisition,
now gtksizegroup.c:do_size_request() fires the signal first and allows it
to overflow the minimum/natural sizes returned by extended layout
implementations.

GtkWidget->get_natural_size() is now left assigning default -1 values to
ensure they both get overridden by the size-request handling.