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Add ListBox.set_scrollpos() and a wheel mode that scrolls the view (#1616)

* Add ListBox.set_scrollpos()

Scroll a ListBox view to an exact row, as Scrollable.set_scrollpos does.
It is the base for a ListBox wheel mode that scrolls the view.

- The focus stays while one of its rows is in view; otherwise the first or
  last widget in the view takes it.
- A pending set_focus() or set_focus_valign() placement is completed first,
  so the next render keeps the requested row.
- ScrollBar keeps scrolling a plain ListBox by its focus: it skips
  ListBox.set_scrollpos, and still calls a subclass's own set_scrollpos.

* Add a ListBox wheel mode that scrolls the view

ListBox, TreeListBox and Table take wheel="focus" | "view".
The default "focus" keeps moving the focus one row per notch.

- "view" scrolls the view one row per notch; the focus stays while one
  of its rows is in view, otherwise it moves as set_scrollpos moves it.
- A notch measures only the rows around the view, so its cost does not
  grow with the body, and unsized or wrap-around walkers work too.
- When the view cannot move one row, as when the cursor of the focus
  widget keeps it in place, the notch does what "focus" mode does.
- At either end of the list the notch is returned unhandled in both modes.

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