Sets the adjustment to be used as the "model" object for this range
widget. The adjustment indicates the current range value, the
minimum and maximum range values, the step/page increments used
for keybindings and scrolling, and the page size. The page size
is normally 0 for GtkScale and nonzero for GtkScrollbar, and
indicates the size of the visible area of the widget being scrolled.
The page size affects the size of the scrollbar slider.
