Now that we automatically set up the focus_vadjustment we only
need the adjustment to handle the page-up/down scroll length correctly.
We use parent_set to automatically pick up the adjustment from a
scrollable (i.e. viewport) parent which makes add_to_scrolled useless, so
it can be removed.
Unhighlight rows if they are removed.
Drop the automatic drag motion based auto-scrolling. This is not really
right, as it e.g. hardcodes auto-scroll regions and assumes we're the only
widget inside the scrolled container. Its also imho in the wrong place,
autoscrolling if any should be done on the scrolled-window or viewport.
All the removed code is implementable outside Gtk+ though.
