improving web text editor: geo click now inserts into editor

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@30393 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
refactoring TextPreviewPanel

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@30394 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e