Re-implement 'SceneInspector' as a hierarchical viewer.

[bentoogood]

To help with display of more complex attributes attached to entities.

Some minor UI improvements to come along with that:

• two tone row striping should stretch width of view
• given the importance of 'type' it should be listed immediately after 'Node','Path','Bound'. Type should be cheated for entities that have a transform but no object to say 'type: Group' (?)
• horizontal anchor for left edge of second column should be quite a bit further right. This is to avoid the columns jumping around when the longest label in the first column changes (due to selection change).

[andrewkaufman]

We're a bit confused about this ticket. @johnhaddon do you think we should keep this one? I'm sure the UI of the SceneInspector needs a lot of work, but I'm not sure about the suggestions here and the title.

[johnhaddon]

Keeping it open makes sense to me - as we add more and more stuff there we will want sections to be collapsible/hierarchical and we will want to take account of each of those items. I think when we first made the ticket we might have been thinking of using a tree view, which I don't think is the right approach now (we need a richer set of widgets for different representations), but since that isn't mentioned explicitly I think we should keep it.

[johnhaddon]

I've broken out the second item from the todo list into #897, since it's unrelated to the basic restructuring of the UI into a hierarchy.