Do not write a manifest entry for the "Thumbnails" directory

By § 4.3 of ODF 1.2 Part 3:
"For directories, the manifest file should contain a <manifest:file-entry> element only if a
directory contains a document or a sub document. See 3.1. A directory for administrative or
convenience purposes, such as a directory that contains various unrelated image files, should not
have an entry in the manifest file."

Roundtrip tested with LO 3,5 and Calligra Words