Always use navigateToURL to open a browser window.

This trick was great back in the day, but I can't find any modern
browsers that still need it from my brief testing: IE, Safari, Chrome,
Firefox. It even has the inverse effect in IE11 (popup blocked using
ExternalInterface, but navigateToURL works) and crashes Safari (it blows
up trying to serialize the return value to Flash?)... so just retire it.

Fixes #183.

@tconkling: I remember you adding this same anti-block to aspirin way
back. Do you know if it's still useful anywhere?