Added missing import in startup/gui/viewer.py.

This hadn't been seen before because the order config files are loaded in by IECore.ConfigLoader is non-deterministic.
Reimplemented SceneInspector to allow further development.

- Replaced single giant label with HTML with hierarchy of Widgets. This will allow much more customisation and interaction.
- Made sections collapsible.
- Introduced tabs to separate major sections.
- Added mechanism for registering custom sections.
- Added Diff and TextDiff classes to simplify the display of diffs.
- Added Row class to simplify creation of standard inspector rows.

Fixes #36.
Fixes #821.