Don't preventDefault on mousedown events

This prevents focus/blur events from getting dispatched,
which was the root cause of #1295.

Fixes #1295.
Do preventDefault behavior on ⌘Z and ⌘⇧Z

This prevents some instances of https://bugs.webkit.org/show_bug.cgi?id=117092,
and generally we don't ever want the default undo
behavior when these handlers are triggered.
Remove unused