Merge pull request #931 from johnhaddon/sceneInspector

Another batch of SceneInspector improvements
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.
More icons.
wip
icon tweaks
wip