uobject: Optionaly disable the changed? <-> UVar bridge.
uobject: Optionaly disable the changed? <-> UVar bridge.

This bridge costs one at per UVar, which is too much with current
implementation on some platforms.

	* share/urbi/uobject.u (UVar.hookChanged): Disable changed?
	hook if set to false.

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