#1020 support thumbnailer for directories

[AL89]

Hi,
I tried to create a custom thumbnailer for directories (mime type inode/directory), just to find out that it doesn't work in pcmanfm, although it works fine in nautilus.
I looked at the code, and it turns out that fm_file_info_can_thumbnail() requires the target object to be a regular file.
With a simple 1-line change (as in the attached patch), directories can be allowed as well, and my thumbnailer works as expected. I hope it can be merged for the next release.

[AL89]

Hi,
I made a small update to create thumbnails of directories even when they have 0 size. Is there any chance that this patch is included in the code?

If you are interested, my thumbnailer for directories is here: https://github.com/livanh/directory-thumbnailer