Adds a control (typically a button) to a list trigger controls.
See the Triggers property for information on what triggers are. This method is primarily for situations where the see cref="Triggers"/> property can't be used because the ID of the trigger control is not known until runtime (e.g. for controls in Repeaters). Controls added via this method are not maintained as part of this control's ViewState. That means that if you use this method, you will need to call it for each request, not just non-postback requests. Also, you can use both this method and the Triggers property for the same control.