UpdatePanelAnimationExtender - No animation when an update is initiated by a control inside UpdatePanel (ChildrenAsTriggers)

[AlekseyMartynov]

See sample and description in #162

[AlekseyMartynov]

Technical details:

The _triggerControlsClientID array contains only IDs of controls which are specified in the <Triggers> collection. Possible solution is to analyze whether _activeElement is located inside the UpdatePanel to which UpdatePanelAnimationExtender is attached. However this functionality would be new, and should be controlled by an additional option. I mark this ticket with the "enhancement" label.