Add symbolic constant in place of literal '1'.
Scrolling with mouse wheel takes wheel turn rate into account.

Previously, scrolling occurred at a fixed speed which felt too slow
whenever you really give the wheel a good flick.
f symbolic constant
Grid<T>.reset reuses existing HTMLTableRowElement objects.

Fixes jumping seen while scrolling.

Can do more of this if WindowView<T> uses move instead of remove/insert.
Change of window offset triggers 'reset' when move is > half of window size.