removeProperty: warn about non-existing properties.
removeProperty: warn about non-existing properties.

	* src/object/object.cc (warn_hard): New.
	Use it to warn about non-existing properties.
	* doc/specs/object.tex: Fix sorting error.
	Document this change.
	* doc/tables/release-notes.tex: Ditto.

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