add support for static classes analogue to abstract classes

[PaulDebus]

Hey,

I already posted an feature request issue on your github project page. Here is the pull request so you can easily include the changes.

With kind regards Paul Debus

[hmenke]

Merged at new upstream: https://github.com/pgf-tikz/pgf-umlcd/commit/93b962f977d0589eb040f29a4a4a56ae63d59124