Org reader: respect unnumbered header property

Sections the `unnumbered` property should, as the name implies, be
excluded from the automatic numbering of section provided by some output
formats.  The Pandoc convention for this is to add an "unnumbered" class
to the header.  The reader treats properties as key-value pairs per
default, so a special case is added to translate the above property to a
class instead.

Closes #3095.

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