Implement a picker for selecting an item class, using the editable string picker.

Used in special node editing and also item editing.

rsrc/schemas/scenario.xsd

@@ -373,6 +373,15 @@
 					</xs:simpleContent>
 				</xs:complexType>
 			</xs:element>
+			<xs:element name="item-class" minOccurs="0" maxOccurs="unbounded">
+				<xs:complexType>
+					<xs:simpleContent>
+						<xs:extension base="xs:string">
+							<xs:attribute name="id" use="required" type="xs:integer"/>
+						</xs:extension>
+					</xs:simpleContent>
+				</xs:complexType>
+			</xs:element>
 		</xs:sequence>
 	</xs:complexType>
 	<xs:element name="scenario">