Blender Git Loki

Use custom collection property to pass the asset list to UI lists

`UIList` is designed to use collection properties. I previously did a
whole bunch of hacks to use it without. This isn't acceptable for
master, instead a proper collection property should be provided.

Idea is to let scripts register a custom collection property, that the
asset view template can fill with the latest asset list contents and
pass that to the UI list.

Longer term I'd prefer if the UI code would support other containers,
and not rely so much on RNA and custom/ID properties. I have ideas and
plans for this, but meanwhile, this will do to remove the hacks.