Blender Git Loki

Outliner: Draw collection color behind icon

Rather than coloring the icon itself and drawing a bar to the left, draw
a roundbox behind the collection icon. This has the benefit of being
small but still easily noticed.

Two downsides that need to be addressed:
1. The active collection highlight is not shown. This needs to be
refactored anyway for all types because it is not always easy to tell
which items are active.
2. Hierarchies are not colored.