Split reactor POM into separate parent and reactor POMs

[Bananeweizen]

Currently the POM in the root of the project is used both as reactor (or aggregator) and parent for the Maven modules. That's done in many projects, but leads to a minor annoyance: All plugins listed in this POM are executed for all modules and the aggregator itself, it's not possible to have a plugin in the aggregator ("at the end") only. By splitting the parent POM out into a separate file that would be possible, and we could avoid such things like the newly introduced profile with file existence check from #615.

Does anyone see a reason why NOT to split out the parent POM content into a separate file? E.g. moving it into reposityroot/parent/pom.xml? Maven will be happy with both, but I don't want to break anything else on CI that I might not be aware of.