Blender Git Loki

Use custom .py property for active asset index, stored in the workspace

Previously I just used the active color index of the first palette to
get things to work.
I would've preferred if the asset view template could register own
properties and store them in the `UIList` it creates. That way you could
have multiple asset views with entirely independent data. But since this
isn't possible, we need a different way to store such data, I think the
workspace makes sense. It should still be possible to store different
data for different use-cases, e.g. to show a pose asset list in pose
mode and a material asset list in object mode. So idea is to let
scripts/add-ons register custom properties for their specific use case
(e.g. "Active Pose Asset" for the pose libraries).