thumbnailer: Correctly cleanup stale thumbnailer directories

Fixes #79 (closed)

Current thumbnailer logic is not really designed to support the generation of directory thumbnails. Most notably the cleanup is done with g_unlink which will fail in the case of directories. Scope of this merge request is to add this functionality.

Comments

Please review carefully. I am everything but an expert in C. My only way to test this change locally was to run Nautilus with a copy of the modified files. Happy to receive feedback.