use installed qcad fonts if PyCAM's fonts are not found (useful for later debian packaging - excluding the font files)
respect a PYCAM_FONT_DIR environment variable for locating font files


git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@853 bbaffbd6-741e-11dd-a85d-61de82d9cad9