Build version and all that stuff

[tngreene]

Right now all we have for version numbering is what is in init.py. XPlane2Blender has this extremely complicated data model vs build number system that has caused more headaches that it has really ever been worth.

I think for this version we can get away with simply well defined use of Blender's addon version, and some custom flag saying when it was built and if it is alpha,beta

So, no DATA_MODEL property this time - since .for is a very very simple spec.

• Unit test showing we have "YYYYMMDDMMSS" or "NO_BUILDNUMBER"
• Unit test showing we have "dev", "alpha", "beta", "rc"

No legacy file!

TODO: Review spec to make sure our ordering of the VerStruct stuff makes more sense this time.

Also to do, can we do without the whole version history thing? This, was, again, very complicated and so far for XPlane2Blender I've never had to do more than see the last version they used (which was either a way old version or the previous version)