Custom thumbnail support

It would be very cool if Thunar / Tumbler could support setting custom thumbnails.

This could be done in a couple of ways. What I'm using right now (https://github.com/j-james/thunar-folder-thumbnails) checks for a .folder.* file, and sets a thumbnail based on that. This has the benefit of instantly working, but is a little bit janky and doesn't work for files (if custom thumbnails on files would even be desired - I personally don't see any use case).

If this was added, it would also be nice to make thumbnails settable within Thunar, preferably by clicking on the icon under properties.