[TIMOB-25522] iOS: Expose navigation-window instance property to Ti.UI.Window

In a production app, there are many cases where the developer would like to know the navigation-window context he/she is currently in. For example, when going through a set of windows to select a configuration, the developer calls "openWindow(nextWindow)) to open the next window ("push it") into the current context. Right now, developers either store a global reference, e.g.

Alloy.Globals.navWindow = nav;

-> [BAD] or pass the navigation-window to each new window being created, e.g.

Alloy.createController('nextWindow', {  
  navigationWindow: nav 
});

-> [BAD] I am proposing a solution where the developer can reference the current navigation-window using the "navigationWindow" instance property. See the following test-case for an example:

var win = Ti.UI.createWindow({
    backgroundColor: '#fff'
});

var nav = Ti.UI.iOS.createNavigationWindow({
  window: win
});

var btn = Ti.UI.createButton({
    title: 'Open next window'
});

btn.addEventListener('click', function() {
    win.navigationWindow.openWindow(Ti.UI.createWindow({
      title: 'Next Window!',
      backgroundColor: 'green'
    }));
});

win.add(btn);
nav.open();

While the developer could still just use the "nav" property, it will get more complex when using a multi-level application which scope changes during the navigation-flow. Ideally, once we move the navigation-window to the universal Ti.UI.NavigationWindow namespace, we could expose the same for Android (cc [~jquick]) and Windows (cc [~kiguchi]). Requirements of this API: * Return existing references only, no copies * Make the property read-only * Only return a navigation-window if one exists, error else