Supply canonicalPath to thumbForDirectory instead of fake QUrl
`url` is either a `thumbnail:` URL or a local URL (when called from the m_propagationDirectories recursion), in both cases only the local path is relevant. Always supply a local path to the thumbForDirectory method, this also avoids creation of a temporary QUrl. Make the path canonical, as the path will be used will be used for determining the thumbname hash and the Thumb::URI metadata field. Otherwise, the directory thumbnailer will create duplicate thumbnails for e.g. symlinked directories: https://specifications.freedesktop.org/thumbnail-spec/thumbnail-spec-latest.html#THUMBSAVE > You need the absolute canonical URI for the original file, > as stated in URI RFC 2396.