Fixes #215: validator: build additional plain jar file, attempt 2

[mistmist]

Argh, accidentally wrote archiveClasses not attachClasses as intended.

[Asbjoedt]

With this PR is the ODF Validtor now available as a library to use in the POM file?
If so, how can the ODF Validator be imported?
i.e. this does not work import org.odftoolkit.odfvalidator

This resolves correctly in the POM file:

<dependency>
      <groupId>org.odftoolkit</groupId>
      <artifactId>odfvalidator</artifactId>
      <version>0.12.0</version>
      <type>pom</type>
</dependency>

[svanteschubert]

The reuse by Maven artefact worked earlier already, see
https://mvnrepository.com/artifact/org.odftoolkit/odfvalidator/0.12.0
I usually do not use the pom type in the dependency, the detailed documentation is here:
https://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#Dependency_Management