Make Onyxia fly and avoid movement during cast

Change the InhabitType of Onyxia from 3 (Ground and Water) to 5 (Ground and Flying) and make her flying on the player's screen by using SetHover(bool). And avoid movement during cast by the verification of the presence of the UNIT_STATE_CASTING flag.

known bug : Onyxia has a strange fly on the screen when she is flying up and when she stays in the air without moving.