Add the CTF management code from the demo/volume-sandbox branch

This continues the steady march of pulling in features from the experimental `demo/volume-sandbox` branch. This replaces the save/load context menu actions in the CTF editor with a `CtfManager` class which allows a more user-friendly CTF-wrangling UI to be built.
Right. The CTF editor code always uses the same `keyfunc`. So you want me to change the default to `itemgetter(0)` so it doesn't have to be passed everywhere?

`set_delta` is a method in `AlphaFunctionEditorTool`, and `ColorFunctionEditorTool`. Both of those classes are abiding by the convention used by the CTF editor for its piecewise functions. I don't see a problem with the assumption being made there.