not on screen starting from the oldest generated one. Rules (aka memory
profiles) are simple by now, but they work as expected.
Plus: added functions for getting Linux total memory and free memory by
querying the /proc filesystem. Generator creation by mimetype. -Fixes.
BROKEN: thumbnail, presentation (need to reimpl a virtual method)
TODO1: notify generators when cancelling jobs.
TODO2: add forward caching.
TODO3: rationalize code, clean it up and update README.png
TODO4: periodically check for free memory and unload pixmaps if needed.
TODO5: wisely choose default values for memory profiles.

svn path=/branches/kpdf_experiments/kdegraphics/kpdf/; revision=372514