Blender Git Loki

Outliner: Element mouse hover feedback

Some little UI polish to get familiar with outliner code (but also because it's a useful feature). Committing to blender2.8 branch but can also port to master (2.7) if wanted.

This basically causes the mouse hovered element to be highlighted. Contrast of the highlight should be fine, even with a non-default theme. Also did some minor cleanup.