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.