Added support for multiple values per cron field (comma separated cron fields).
Multiple values per cron field are now stacked vertically. They're sorted according to the order defined in the select element.
Clicking outside of the select dialog now closes all open dialog boxes.
Clicking on a label that's not associated with the currently opened dialog box, will close the currently opened dialog box. This required the concept of a globally 'active' dialog box.
Removed the 'save' control and AJAX callback. I think this should be taken care of externally.