This implements a new Page scrolling mode, essentially bringing (and extending) the functionality from `PDFSinglePageViewer` into the regular `PDFViewer`-class. Compared to `PDFSinglePageViewer`, which as its name suggests will only display one page at a time, in the `PDFViewer`-implementation this new Page scrolling mode also support spreadModes properly (somewhat similar to e.g. Adobe Reader).
Given the size and scope of these changes, I've tried to focus on implementing the basic functionality. Hence there's room for further clean-up and/or improvements, including e.g. simplifying the CSS/JS related to PresentationMode and implementing easier page-switching with the mouse-wheel/arrow-keys.
