Schema: Promote [object]duration= key to non-ActionWML objects

It's used by the object auto-expiry mechanism, so it's meaningful
even outside of ActionWML.

data/schema/core/actionwml.cfg

@@ -375,7 +375,6 @@
 		{INSERT_TAG}
 		{DEFAULT_KEY take_only_once s_bool yes}
 		{DEFAULT_KEY delayed_variable_substitution s_bool no}
-		{SIMPLE_KEY duration object_duration}
 		{SIMPLE_KEY silent s_bool}
 		{SIMPLE_KEY image string}
 		{SIMPLE_KEY name t_string}

data/schema/units/modifications.cfg

@@ -253,4 +253,5 @@
 	name="object"
 	max=infinite
 	super="units/$modifications/base"
+	{SIMPLE_KEY duration object_duration}
 [/tag]