Added gtk_extended_layout_is_height_for_width()

Added gtk_extended_layout_is_height_for_width()

Added an indicator telling whether a widget prefers to be allocated
as height-for-width or width-for-height. Usually this depends on the
orientation of a container or the nature of a content widget like GtkLabel.

This indicator is only used in the seldom case where a parent is allocating
free space to the child and the child can flow in either direction, GtkWindow
and GtkScrolledWindow are users of this api.