I believe this is an issue between Mac and Windows, where doing this is a workaround to how the OS automatically aligns lists like this. I think 1 can be hacked but it's difficult with the other. Hence it's not implemented for consistency reasons.
A completely custom list may be required, like Photoshop layers and groups for example. I like the idea of gradating the background somehow to show what's contained in one folder.
James Badcock
Graphisoft Senior Product Manager