memory reduction on the displayed page. OutputDev under big changes.
Now DocumentObservers have an unique ID so they can queue requests to
documents and they get their data stored in Page(s). No more Pixmaps or
Thumbnails requests, every observer can request a custom-sized pixmap for
a given page. That makes room for new observers (like a cool perspective
book like page viewer or stacked viewer or what your fantasy suggests :-).

svn path=/branches/kpdf_experiments/kdegraphics/kpdf/; revision=346792