listview: Only allocate necesary rows

listview: Only allocate necesary rows

This is the big one.

The listview only allocates 200 rows around the visible row now.
Everything else is kept in ListRow instances with row->widget == NULL.

For rows without a widget, we assign the median height of the child
widgets as the row's height and then do all calculations as if there
were widgets that had requested that height (like setting adjustment
values or reacting to adjustment value changes).

When the view is scrolled, we bind the 200 rows to the new visible area,
so that the part of the listview that can be seen is always allocated.