Add e.returnValue in addEventListener('beforeunload') for sake of Firefox
Add e.returnValue in addEventListener('beforeunload') for sake of Firefox

git-svn-id: e52e7dec99011c9686d89c3d3c01e7ff0d333eee@2680 eee81c28-f429-11dd-99c0-75d572ba1ddd

chunchill/svg-edit,luqin/svg-edit,luqin/svg-edit,luqin/svg-edit,svn2github/svg-edit,djyaron/svg-edit,djyaron/svg-edit,luqin/svg-edit,djyaron/svg-edit,chunchill/svg-edit,svn2github/svg-edit,djyaron/svg-edit,chunchill/svg-edit,chunchill/svg-edit,svn2github/svg-edit