(Observers)Fix memory issues during observer iteration

(Observers)Fix memory issues during observer iteration

[Issue#]   N/A
[Problem]  Crash could occur when adding a previously removed actor back into the scene
[Cause]    Observer classes were using SceneObjectRemoved callback to remove themselves
from that object's observer list while the object is in process of iterating over the list
[Solution] Mark object as disconnected from scene and remove observer later

Change-Id: I552017a0d8284973f565cd08275f1270fc8e9668
Signed-off-by: David Steele <david.steele@partner.samsung.com>