POM: bump to pom-imagej 2.6
POM: wrap Eclipse lifecycle-mapping in a profile

Otherwise, annoyingly, the Maven build yields errors about
lifecycle-mapping not being found in Maven Central.
POM: override inceptionYear for license blurbs

The original ImageJ 1.x was founded in 1997, but that version is in the
public domain. The 2.x version began in 2009 and hence the copyright
date range starts then.