Fix delegating focus/blur events in IE

These events don't bubble natively in any browser, so we used event
capturing to provide event delegation support for them. This workaround
doesn't work in IE, however.

This translates focus/blur events to focusin/focusout, which bubble
natively.

A potential pitfall of this change is if a user inspects `event.type`
inside an event handler for "focus", for instance, they will get
"focusin" back.

References #869, fixes #868