Add None arrow type to prevent arrow and always center on screen like a ...

...popup dialog

This does not implement the special present method that does NOT take a UIView to anchor to, but it does add in the None arrow type so the arrow is never drawn and the dialog is centered on screen.  So basically, the provided rect and InView in the present method is not used when the arrow direction is None.  It would be better to have a special presentAsPopupDialog method that just takes an bool animated argument, so this diff is more of a suggestion.