Add support for enumerations

Some attributes can only have a predefined set of values. It would be nice to have autocomplete for these values.

Example
```xml
<xs:simpleType name="ParameterModeAttributeValuesType">
  <xs:restriction base="xs:string">
    <xs:enumeration value="INPUT" />
    <xs:enumeration value="OUTPUT" />
    <xs:enumeration value="INOUT" />
  </xs:restriction>
</xs:simpleType>
```