Add a new "rechargeable" flag to items

A way to set this flag is not yet exposed in the scenario editor.
The flag is intended only for non-stackable items, but this currently isn't enforced.
Items now have a maximum number of charges, which is equal to their default number set in the item record.
Enchanted items with charges are now rechargeable.

test/files/items/full.xml

@@ -28,8 +28,9 @@
 			<cursed>true</cursed>
 			<concealed>true</concealed>
 			<enchanted>true</enchanted>
+			<rechargeable>true</rechargeable>
 			<unsellable>true</unsellable>
 		</properties>
 		<description>This is a silly, silly description.</description>
 	</item>
-</items>
+</items>