Add support for Process Definition development version

In many development and release processes a process definition (as an artifact) passes through different testing, staging, UAT, production, etc. environments. In each of these stages something wrong could be found and a new version of the process definition to be needed.

As all the mentioned environments are using, naturally, different workflow engine databases. This means that the same development version of the process definition might (and usually do) have different deployment version in each environment, i.e. the deployment version in the most cases does not match the development version. Thus, when an issue or a support request reaches the process definition developers they do not know which version of the process definition in the source repository to look at.

To overcome this problem:
- create an XML extension to hold the development version of the process definition;
- extend the engine API to expose (read only) the development version of a deployed process definition;
- extend the (Eclipse) Designer to support (read and write) the XML extension.

Note: This issue is based on the forum discussion https://forum.flowable.org/t/support-for-process-definition-development-version/247

Beginne damit, das Prozessdefinitionsmodell und die Deployment-API nachzuverfolgen, und untersuche anschließend die Erweiterungspunkte des Eclipse Designer; im Issue werden keine bestimmten Dateien oder Tests genannt. Als erledigt gilt die Aufgabe, wenn eine XML-Erweiterung für die Entwicklungsversion vorhanden ist, die Engine sie für bereitgestellte Definitionen schreibgeschützt bereitstellt und der Designer sie in den beschriebenen Umgebungen lesen und schreiben kann.

Vom Indexierungsmodell aus dem Issue-Text verfasst.