Symbolic I/O for more enums

- Nearly every enum that gets written to a file now uses a symbolic form rather than a numeric form. Input supports both forms.
- The special node type enum, however, no longer has a symbolic form output operator, as the only place it's output is in the special nodes file which uses the opcode.
- Collected some enums scattered around the files into one place in simpletypes.hpp

rsrc/schemas/terrain.xsd

@@ -144,7 +144,13 @@
 				<xs:element name="special">
 					<xs:complexType>
 						<xs:sequence>
-							<xs:element name="type" type="xs:integer"/>
+							<xs:element name="type">
+								<xs:simpleType>
+									<xs:restriction base="xs:string">
+										<xs:pattern value="[a-z-]+"/>
+									</xs:restriction>
+								</xs:simpleType>
+							</xs:element>
 							<xs:element name="flag" minOccurs="0" maxOccurs="3" type="xs:string"/>
 						</xs:sequence>
 					</xs:complexType>