Prevent event thread from picking up notifications too soon

Prevent event thread from picking up notifications too soon

[problem]      Animation-finished and property-notifications can be fired off too soon.
[cause]        We queue the message from update straight away, even before the scene has been
               updated.
[solution]     Create another buffer in the notification-manager which is only swapped at the end
               of the update.

Change-Id: I7b72fa2485ed9faa606b52a3925e111df2931776
Signed-off-by: Adeel Kazmi <adeel.kazmi@samsung.com>