Added icons for upcoming SceneInspector features.
Simplified SceneInspector Row class and added a DiffRow.

The Row wasn't general purpose enough and now just contains the bare minimum needed for alternating colours. The DiffRow always displays a diff, and will be specialised to provide features specific to the SceneInspector.
Introduced Inspector abstraction to SceneInspector.

An inspector is simply a callable which can suck a value out of a Target. By passing these around we have abstracted out the inspection process enough to start performing queries on a targets ancestors and upstream nodes, to provide more advanced SceneInspector features.
Added removeOnClose argument to Window.addChildWindow() method.

It's becoming a common enough pattern that making it as easy as possible makes sense - otherwise it's too tempting to be lazy and just leave the windows dangling.
Added attribute inheritance diagramming to SceneInspector.

This allows the user to see how an attribute has been inherited through the hierarchy to the inspected path, with each change of value being shown in diff form.