POM: add developer profiles for SNAPSHOT pinning

These handy profiles automatically update all version properties for
a given set of projects to SNAPSHOT versions rather than releases.

This makes it easier to develop multiple projects against one another,
although you must still be careful not to push changes to master which
rely on the SNAPSHOT coupling (otherwise the Jenkins build will fail).
pom.xml: add new property anchors

As there are now property tags in the profiles section, there is not a
good anchor point to restrict the search and replace range for scripts
like bump-pom-scijava.

This adds a unique comment at the start and end of the primary
properties section to distinguish it from profile properties.