Use single qualifier for all built artefacts in Maven build

We would like to be able to build documentation by not using sources from only one plugin, but also from other plugins. Since plugin qualifiers are based on the Git timestamp of the last commit of the plugin, this would not take into account sources taken from other plugins. We could thus violate the 'same version implies same content and hash' constraint. See also #73 (closed) and #73 (comment 10935) for more information and discussion.

The idea is to:

• Use fixed qualifiers, rather than different ones for each plugin/feature/etc based on Git timestamps.
• Determine for the entire Git repo the timestamp of the last change.
• Use that single timestamp to determine the qualifier.
• Configure Maven build to use that single qualifier.

This would solve the described issue.

It would also lead to every release having new qualifiers, but we already step either the major, minor or micro version anyway, so the qualifier is a bit redundant already. As such, 'unnecessary' qualifier increases are not a problem for releases. For development builds, we could opt to change it as well, and add -dev to the qualifier. Similarly we could add -M1 and -RC1 etc for non-final releases.