m2eclipse-mavenarchiver writes pom.xml/pom.properties too aggressively

The AbstractMavenArchiverConfigurator.writePom method is called whenever
the Maven builder is called. The method writes pom.xml/pom.properties
archive files no matter if pom.xml has been changed or not.

The PR fixes the issue by calling the writePom method only if pom.xml
is changed or if there isn't pom.xml in the output folder.