Fixed SceneInspector update.

Previously it made no attempt at all to update when a plug it was inspecting was dirtied. Now it does.

Fixes #176.
Improved SceneInspector shader representation.

It now displays the name of the terminating shader in the network rather than an obscure "ObjectVector..." string.

Fixes #147.
Removing todo.
Improved SceneInspector numeric formatting.

It now matches the formatting performed by the NumericWidget.

Fixes #88.
Merge pull request #182 from johnhaddon/sceneInspectorImprovements

Scene inspector improvements