Added RadialPlainLayout

The "user experience" around the radial layout needs to be improved
though.
What is fun is adding just one child to the rootnode and then a loooot
of children to that one... and then just one more child to the rootnode
and the layout-animation (in short-animation mode) will look cool,
somewhat spiraly.

Next is the RadialSmartLayout where i want nodes to expand into white
spaces that are free next to them.