2. Make MainMenu? links call LeftAndMain?_window_unload() to check for unsaved changes before loading new page. Don't register LeftAndMain?_window_unload with window 'beforeunload' event because it
cannot stop the page unload and the save check is now done via links (where stopping the unload works).
3. Make external Logo link open in a new window like the Menu-help link to prevent unsaved changes being lost.
More info: http://www.elijahlofgren.com/silverstripe/alert-users-when-leaving-tab-with-unsaved-changes/ (merged from branches/gsoc)
git-svn-id: svn://svn.silverstripe.com/silverstripe/open/modules/cms/trunk@41798 467b73ca-7a2a-4603-9d3b-597d59a354a9