Registration mechanism for custom column in SceneHierarchy

[andrewkaufman]

This would allow external additions of display information specific for the user. Relates to #48 and #80.

[johnhaddon]

It would be handy to have examples of the sorts of things people will want to display here. I'll start us off with a couple :

• Visibility
• Shader assignment (maybe colour coded?)

[johnhaddon]

As we add features to the SceneHierarchy, we're going to need to keep a close eye on performance. The SceneHierarchy is one of our worst performing UI elements, because it actually deals with every item in the scene (when expanded), and does so using Python. It's the one place we access the whole scene without using C++ to do it. At some point I think we'll need to reimplement our Gaffer.Path class in C++, and then reimplement the PathListingWidget _PathModel() in C++. That will have some interesting problems, like how do we write C++ Qt stuff, and get it into Python for both PyQt and PySide. And it'll also bring with it more fun build permutations, because we'll need to build and link with the right Qt for each host application, instead of just worrying about it at the Python level...

[yutapple]

@markwendell was asking for this too :)

[yutapple]

aannnd would be great if it grays out when visibility attribute was set to hidden :)