[TIMOB-17325] iOS: Allow Window's hideShadow to work on nav bar without requiring barImage to be set.

The "[hideShadow](http://docs.appcelerator.com/titanium/latest/#!/api/Titanium.UI.Window-property-hideShadow)" Window property was introduced with Ti 3.2, but only works if "barImage" is specified. This leaves no way to disable the 1px shadow from the navigation bar if we are using a solid color, or an iOS 7 blurred translucent navigation bar. As shown on the two attached examples below, the shadow can create an undesired "look" on an iOS 7 translucent nav bar. Using a transparent image for barImage does not work, as it completely removes the translucent blur effect from the navigation bar. It would be great if the requirement for barImage is removed, so we can disable the navigation bar when needed, regardless if the bar has an image, blur translucency or set to a solid color.