Show plaintext preview in AV before rendering LaTeX

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@25229 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
Fixed last commit

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@25230 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e