Implement loading of scenarios

- This also tweaks the scenario schemas for consistency between schemas and code, adds some unique key restraints, and makes some attributes required.

rsrc/schemas/items.xsd

@@ -120,6 +120,11 @@
 			<xs:sequence>
 				<xs:element ref="item" minOccurs="0" maxOccurs="unbounded"/>
 			</xs:sequence>
+			<xs:attribute name="boes" type="xs:string" use="required"/>
 		</xs:complexType>
+		<xs:unique name='itemIds'>
+			<xs:selector xpath='item'/>
+			<xs:field xpath='@id'/>
+		</xs:unique>
 	</xs:element>
 </xs:schema>