Fixes issue #61

Since FLAC files can contain covers, it makes sense
to parse their metadata

This should close #74

Use color palette an size according to
https://developer.gnome.org/hig/stable/icon-design.html.en

Relates 0a2a398c

We didn't take the whitelist into account while
removing dangling files from [Content_types].xml

- Replace some class members by instance members
- Don't thread the cleaning process anymore for now

This should fix the intermittent failures, thanks
to @pollo for the tip

MS Office XML rsid is a "unique identifier used to track the editing session
when the physical character representing this section mark was last formatted."

See the following links for details:
- https://msdn.microsoft.com/en-us/library/office/documentformat.openxml.wordprocessing.previoussectionproperties.rsidrpr.aspx
- https://blogs.msdn.microsoft.com/brian_jones/2006/12/11/whats-up-with-all-those-rsids/.

In XML, the order of the attributes shouldn't be meaningful,
however, MS Office sorts attributes for a given XML tag
differently than LibreOffice.