Added support for specifying a SearchClassSelector() method on ModelAdmin, which returns 'tabs' or 'dropdown', to enable a better UI for ModelAdmins that manage a large number of different classes.
Updated ModelAdmin's javascript to provide more status indicator (loading icons, success/failure messages), using HTTP status codes and custom status text.
git-svn-id: svn://svn.silverstripe.com/silverstripe/open/modules/cms/trunk@62334 467b73ca-7a2a-4603-9d3b-597d59a354a9