switch to image asset for icons and spashscreens


git-svn-id: https://geuz.org/svn/gmsh/trunk@18720 59c8cabf-10d6-4a28-b43f-3936b85956c4
git-svn-id: https://geuz.org/svn/gmsh/trunk@18721 59c8cabf-10d6-4a28-b43f-3936b85956c4