Make the private "document layout" of QPdfView public

As other aspects of the QtPdf module, QPdfView still lacks many possible and important features it could theoretically provide, as they are not publicly accessible.

One of them is the documentLayout of QPdfViewPrivate, which would be tremendously useful for knowing the actual visible pages in the viewport or even adding custom objects that do not specifically follow text selections (including portions of images).

By following the QPdfView sources, it is relatively easy to implement a similar function (I recently did one for PySide), but that adds an unnecessary overhead for something that is already done in the underlying code.

I suggest to make that layout struct a public class, and the corresponding internal value retrievable as a read-only property, possibly including a related "changed" signal.