[FEATURE] ux3.Overlay: new closed Event

- as the Popup has a closing animation of 400ms operations on the
Overlay instance in a close handler could lead to problems. Therefore 
a closed event is introduced. This event will be fired after the
animation - when the Overlay is really closed. 

Change-Id: I53cb774f58de9c8e2dd5ad17df81518fc41f4d07
BCP: 0120025231 0000807559 2014
Merge "[FEATURE] ux3.Overlay: new closed Event"