Outliner icons based on model kind

[dj-mcg]

Our users would like a more obvious visual clue as to the different kinds and types of objects in the Outliner. They are specifically interested in visually separating structural/organizational "group" nodes from nodes representing individual USD models. While these are both of type Xform, they have distinct kinds (in our case, "group" and "prop").

One approach that was discussed was to first try to find an icon based on model kind, then fall back to using the model type if one was not found.