Improve event handling for IE

I remembered that it isn't safe to assume `e` is defined on IE and testing if `window.event` exists would be more "failure-proof" since all we want is to block the IE-specific onpropertychange event in some cases.

And since we aren't using the event object for anything else (like blocking certain keys on the keyboard or detecting the node on which the event was triggered), I thought I could simplify my previous suggestion by dropping the event argument.

I have tested this on recent CH, FF, and IE7-8-9-10