It feels slightly wrong to have GtkOrientable operate on widgets, but at
least what happens when an orientable widget changes orientation should
be part of GtkWidget.
This will allow to add more state changes without accessing widget state
from the outside of gtkwidget.c.
This was only living in gtkcontainer.c for historic
reasons. Move it closer to where it belongs, and
rename it from 'idle' to 'layout', since it is
really about the layout phase of the frame clock,
nowadays.
Add a convenience api to skip children
that should not be included in the layout,
and call gtk_native_check_resize on all
native children outside of the vfunc.
