GtkScrollableWindow: Automatically set focus adjustments

GtkScrollableWindow: Automatically set focus adjustments

When adding with a viewport we automatically set the focus
adjustments on the viewport from the scrolled window, so that
when any child widget gets focused we automatically scroll to it.

This is generally nice, but its particularly important for GtkListBox
where focus changing is how we navigate between rows.

We also ensure that the adjustments are always set before adding the
child to the viewport, which we will need later to pick up the
adjustments on add.