595-73 improve navigation/data passing

Improve navigation/data passing within `AddNewItem` page.
 - Use `Binding` to sync view and `FoodInfo` model
 - Use `Navigation.GoBack()` when appropriate
 - Use `PhoneApplication.Current.State` dictionary to pass data to child views
 - Use a Converter to ensure only valid positive integers are saved for quantity

Also, a couple of minor fixes:
 - Fix accept/cancel icons on Datepicker
 - Item look-up now happens when a UPC is typed in (not only when scanned)