Highlight new or extended subsets

[mottosso]

Goal

Let the user know when an asset is being updated or created anew. For example, if rigDefault has been created elsewhere, color the line edit orange. Otherwise, color it green.

Motivation

It can be difficult to know whether you are accidentally updating something that already exists, or creating something new when you meant to update. Especially if working on the same subset in more than a single scene.

Implementation

Apply color to represent existence.