API: Don't export gtk_widget_preferred_size_and_baseline()

API: Don't export gtk_widget_preferred_size_and_baseline()

gtk_widget_preferred_size() is only useful if you want to quickly port a
widget from GTK2 sizing code to GTK3 but does not properly work with
height-for-width as used in GTK. So we don't want to encourage people to
use it. In particular we want people to convert to height-for-width
before adding baseline support to their widgets.