How to change an appointment's custom field values via the "Appointment" context menu (like for labels and statuses)

This example demonstrates how you can add possible values of an appointmen's custom field as additional items into the appointment's context menu, so that an end-user can change the appointmen's custom filed value in a similar manner as for labels and statuses.
The checked state of these additional items is synchronized with an appointment's custom field value in the client-side ASPxMenu.Popup event handler.
The appointment's custom field value is changed on the ASPxScheduler's custom callback after an end-user clicks a corresponding menu item.