Now calling ajax_modifyItem_async when changing a date in the pickadate widget. No error-handling yet.
1. But fix: ajax_modifyItem_async now calls onerror() before throw... Otherwise onerror() is never executed.
2. For the datetime picker in attach callbacks, created a separate function on PickerSet.
3. Now if the picker fails to modify a date, reverting to the previous one from _items. In case there was no previous date, doing a hack: disconnect the callback, clear the picker, reconnect the callback. (There is a function to silently set the date, but no function to silently clear the date -- and this again triggers the callback, which tries another AJAX PUT, which fails once again, and so on ad infinitum...)