Org reader: add :PROPERTIES: drawer support

Headers can have optional `:PROPERTIES:` drawers associated with them.
These drawers contain key/value pairs like the header's `id`.  The
reader adds all listed pairs to the header's attributes; `id` and
`class` attributes are handled specially to match the way `Attr` are
defined.

This also changes behavior of how drawers of unknown type are handled.
Instead of including all unknown drawers, those are not read/exported,
thereby matching current Emacs behavior.

This closes #1877.

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