Blender Git Loki

Property Search: Add "Search match" theme color

We use a highlight on panel headers to convey that they have a search match.
It's also conceivable to use this color to highlight the property editor tabs
that have search results.

Normally we don't like adding new theme colors, but this is quite a special
case. For example, the color in the outliner is green. This patch has a blue
highlight so it feels more cohesive though.

Differential Revision: https://developer.blender.org/D8854