Fixed: in certain degenerate cases, CPTextField may not have a window within _isWithinUsablePlatformRect

Previously, a case could arise where a text field was set as the initial first responder but during cib loading was not yet assigned to a window. In this case _isWithinUsablePlatformRect would fail.

Now _isWithinUsablePlatformRect checks to make sure the text field has a window.
Fixed: CPImageView's _DOMImageElement was not created during initWithCoder.

Previously, _createDOMImageElement was not called until after [super initWithCoder] was called. But that call would end up in CPImageView -setObjectValue, which would attempt to access _DOMImageElement before it had been created.

Now _DOMImageElement is checked before being accessed or created.