Color Transfer Function editor gadget

This big chunk of code implements a CTF editor for creating/editing functions which map from some data space to RGBA values. I'm using it for a volume rendering viewer, but it could also be used for creating a colormap for an image plot.

**NOTE**: This code needs needs traits-enaml [1] and a recent version of enaml[2], but since it's not imported anywhere, the dependency isn't explicit.

1. traits-enaml <https://github.com/enthought/traits-enaml>
2. enaml <https://github.com/nucleic/enaml>