String serialization improvements

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@26206 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
4.9.156.0

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@26207 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e