One additional complication with removing this option from the global `PDFJS` object, is that the viewer currently needs to check `disableAutoFetch` in a couple of places. To address this I'm thus proposing adding a getter in `PDFDocumentProxy`, to allow checking the *actually* used values for a particular `getDocument` invocation.
The `appConfig` contains (mostly) references to various DOM elements, used when initializing the viewer components.
Hence `defaultUrl` seem like a slightly better fit for the new `AppOptions` abstraction, not to mention that it should thus be easier to set/modify it for custom deployments of the default viewer.
The way that various options are handled in the default viewer is currently a bit of a mess (to say the least). Some viewer options reside in the global `PDFJS` object, while others reside in `Preferences`. To make matters worse, some options even exist in both of the two.
Since the goal, with PDF.js version `2.0`, is to reduce our usage of the global `PDFJS` object, we'll instead want pass in the options when initializing the viewer components and when calling API methods (such as `getDocument`).
However given the current state of things in the default viewer, this wouldn't be exactly easy to implement. Hence this patch, which attempts to consolidate the way that viewer (and later API) options are handled by introducing a `AppOptions` singleton that provides *one* centralized way of interacting with the various options in the default viewer.
