Add API to facilitate customizing QPrintPreviewWidget

The QPrintPreviewWidget is currently a black box. If someone wants to customize it to fit their application, this is made difficult by the fact that it depends on internal and private features in Qt.

A specific use case that has been mentioned is for an application that uses WebKit to display documents, and requires the following functionality:
1. Acquire a preview of each page separately from each other, in any format that would let us display it to the user (vectorial preferred over raster, of course).
2. Print any subset of all pages of all documents (eg: page 1, 8, 12 of doc #1, plus page 22 of doc #2), in a single print job.

(see: http://qt.gitorious.org/qt/qt/merge_requests/733)

Currently it seems that this is not supported by Qt, and because most of the relevant API is private, the user has no way to implement support for it themselves. How to achieve this is uncertain at the moment, but once an API has been agreed upon, the creators of the original merge request have stated their willingness to do the implementation.