NSGradient implementation

[Fred Kiefer]

I Just added the gui part of an implementation of NSGradient. This is
based on code by Nikolaus Schaller and still requires some clean up. If
somebody helps me to remove all the remaining FIXMEs in the code I am
willing to define two backend methods for this class and implement them
at least for the cairo backend.

If on the other hand this results in as little feedback as the pattern
colour implementation, I will just forget about that class.

Cheers
Fred