A const reference may be asked. When set all DocumentObservers will be
notified so they can update their gfx if needed. Converted TOC, Links
to the new usage. DocumentViewport can be saved/restored to/from QString
so it can be asked as metadata and saved on document's XML. When loading
a document the viewport is restored exactly where it was when the document
was closed. More fixes with Viewport class. Swapped groupboxes in Perform
dialog. Changed members naming in DocumentObservers and inherited classes.
PageView and ThumbnailsList now linked.

Still regressions / TODOs about the Viewport thing.

svn path=/trunk/kdegraphics/kpdf/; revision=377066