Preview is a  Document-Based Application
see: http://developer.apple.com/documentation/Cocoa/Conceptual/Documents/index.html#//apple_ref/doc/uid/10000006i
for more informations about the document architecture.

Class/Method

main.m :  launch the app
  * crateMenu() set the Main menu

DocumentController : NSDocumentController.m subclass (it is a NSApplication delegate)
infoPanel: => call InfoPanelContoller (Model View Controller)
helpPanel: => call HelpViewer (should be install) for help

Document: NSDocument.m subclass
* NSDocument overide methods

readFromFile:  ofType: => Load the document from a FileName (set the image into the Model => Image.m)
windowNibName: => Return the the gorm file (the View MVC), of the main Window 
windowControllerDidLoadNib:  => Connect UI elements after the Window is load: put the image into the Window and set its  size

* NSDocument private methods
_updateImage => redraw the image with new settings (Zoom)

Defaults : share preview defaults : implement as singleton. 
Can manage maxWindowWidth maxWindowHeight scaleFactor and Alpha (TODO).
observ (Notification) any changes into defaults database (TODO)
	
	
