How to implement cascading ASPxComboBoxes for editing custom fields in a custom Appointment Edit form

This example illustrates a server-side technique of editing hierarchical custom appointment fields using cascading ASPxComboBoxes.
The main idea of implementing cascading ASPxComboBoxes was demonstrated in the following example:
A general technique of using cascading ASPxComboBoxes
In this example, we added two custom fields for appointments (CompanyID and ContactID) and two corresponding ASPxComboBoxes onto a custom Appointment Edit form. 
An approach for customizing the Appointment Edit Form for working with custom fields was described here:
How to: Modify the Appointment Editing Form for Working with Custom Fields
Changing a value of the "CompanyID" combobox results in filtering data in the "ContactID" combobox.