Integrated version support for needs

[danwos]

I'm not sure if Open-Needs shall deal with version or not. As it may be not needed for simple projects

Also the name "version" is not smart, as it is often used by the projects itself to link e.g. a requirement
to a product version.

But we are talking about "documentation version", so the version of the documentation,
which contains the need. I have the feeling that this is more or less an internal var, so that we maybe call
it "doc_version" and it shall be set automatically by each interface when talking with Open-Need Server.