Fixed scroll_to(). Added needed ScrollView method.

Added set_scroll_y method to handle effect updates, which were stale for
programmatic sets of scrollview.scroll_y from ListView.

Got the arithmetic right for optional position argments to scroll_to(),
which give control to placing a view instance for a given index, at a
chosen position within the scrollview container. The default position is
to have the view_instance in the middle, but I can imagine cases where
this will not suffice.

Added several more scroll_to() example calls in list_scroll.py.