gtkscrolledwindow: Add scroll unit handling

gtkscrolledwindow: Add scroll unit handling

On the "scroll" signal, the widget uses
gtk_event_controller_scroll_get_unit() to get the
scroll unit.

When the unit is GDK_SCROLL_UNIT_WHEEL, the
behavior is unchanged: the widget scrolls a
certain number of pixels at each wheel detent
click. This number of pixels is determined by the
window dimensions in get_wheel_detent_scroll_step().

When the delta unit is GDK_SCROLL_UNIT_SURFACE, the
widget directly adds the delta to the number of
scrolled pixels no matter the window dimensions.