Listen for 'scroll' event from scrollbar.
Simplify expression.
Support dragging of scrollbar, keeping DOM element in place throughout.
Updated demo links in README

Name of demo repo has changed.
Include link to version hosted at: https://drewnoakes.com/biggus-griddus/demo/
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.
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.