I find it useful to have a thumbnail for directories in order to aid my memory about where different images are located.
This patch adds another directory view, made up mostly from code copied from "view_dir_list" and "view_file_icons." It displays thumbnails in the directory list of the first image file in each directory. If no suitable file is found, or it cannot create a thumbnail then it displays a standard folder icon.
A side effect of this patch is that it makes the existing folder lists use stock GTK icons, which has the disadvantage of removing the icon for a directory which cannot be accessed (there was no suitable stock GTK icon.) The reason for this is that I needed larger icons for the thumbnail view but am not artistic enough to make large versions of the existing icons look good.
There are probably still a couple of rough edges (eg interaction with the rc file) which need weeding out but I have been using this for a few days and find the feature helpful.
This patch is for 2.0.4, I will make another patch for the devel release in the near future.