Blender Git Loki

UI: New UI list layout type for big preview tiles

This new layout type is meant for the upcoming asset view UI template.
With it it is possible to show big asset previews with their names in a
responsive grid layout.

Notes:
* The layout is only available for C defined UI lists. We could expose
it to Python, but I think there are still some scrolling issues to be
fixed first. (The asset view template doesn't use scrolling for the UI
list.)
* I'd consider this a more usable version of the existing `GRID` layout
type. We may remove that in favor of the new one in future.