UObject: optimize changed! hook code.
UObject: optimize changed! hook code.

Only hook when the first notifyAccess is created, and remove hook
as soon as looping code is activated.

	* share/urbi/uobject.u (UVar.clone): Remove hook code, move...
	(notifyAccess) there. And remove it once activated.

urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi