Fix for #2904 (IAccessible window objects treeted as layout when unavailable).

Fix for #2904 (IAccessible window objects treeted as layout when unavailable).
Specifically:
	* IAccessible NVDAObject's presentationType property: remove all mentions of window objects and implement a new presentationType property on WindowRoot IAccessible NVDAObjects that returns unavailable if unavailable or invisible is in states, layout if the window does not have any extra IAccessibles, and at the very end, content.
	* Move the windowHasExtraIAccessibles method onto the WindowRoot IAccessible NVDAObject.
	* IAccessible NVDAObject's isPresentableFocusAncestor: remove any mention of window objects and  put a isPresentableFocusAncestor property on GenericWindow IAccessible NVDAObjects that is set to False.