refactor: type events to remove need to cast

[haxxmaxx]

• Realized we could (and should) use currentTarget which can actually be typed. This removes most castings to HTMLElement There are a couple exceptions:
  • onContextMenu, since we don't control where the event comes from
  • getElementsByClassName, since the return type is not generic
  • isEventFromColumnAdjuster, since we might move this to common repo
  • native FocusEvent, the target types are hard coded
• Rewrite of copyCellValue. It becomes much cleaner if you just pass the cell element, which is currentElement in the keyboard functions, and we already get from closest in onContextMenu. plus the arg type becomes simpler