Add "listeners".

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl/pkg/rgl@1194 79a1b648-8b68-254f-b20b-c0b50f5137bf
You can't link to a concept, only an alias.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl/pkg/rgl@1195 79a1b648-8b68-254f-b20b-c0b50f5137bf