PixmapRequest class. When requesting pixmaps, one or multiple requests are
sent to the Document that (frees memory as in current policy) and send each
PixmapRequest to the current Generator. Added a signal in generators to
notify the Document when a pixmap generation has finished.

PageView, ThumbnailsList, PreviewWidget have been unbroken after the memory
management commit. (mem management seems in pretty good shape..it's smart.)
Added 'visible widgets' list to those classes to speed up searching and
processing on visible widgets only.

Note: asyncronous pixmap requests can now be queued and we're getting very
close to the threaded generator.

Note2: Leakfixes and memory improvements.

Final NOTE: head merging is possible now, as all remaining work can be
considered bugfixes.. API is getting final. It will only change in xpdf
dep stuff, the already undefined Viewport object and some bits in
Generators.

svn path=/branches/kpdf_experiments/kdegraphics/kpdf/; revision=372787