use proper constant for "geogebra_thumbnail.png"

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31996 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
include 3D thumbnail in ggb files

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31997 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
4.9.245.0

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31998 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e