Port the drawing routines of the DVI plugin to the cairo drawing library.
Cairo can draw into memory buffers, so we don't need to lock the programs
event loop anymore, when we do drawing operations.

Altough cairo is slower then QPainter, the GUI feels more responsive, because the
GUI is not blocking anymore seemingly at random. Also there should still be some
optimisation possibilities in our cairo code.

This change is only relevant for the KDE3 branch of kviewshell, since in Qt4 thepainting engine has the needed functionality.