this introduces the PropertyDrawer node to make it easier to access the
properties associated to a headline - normal drawers don't parse their content
into kv pairs
see https://orgmode.org/manual/Checkboxes.html
We're deviating from Org mode regarding the assigned css classes but the chosen
classes feel better than (on, off, trans) and it's not like the export matches
1:1 otherwise.
Org mode does not care where those tokens are when it comes to the
export (afaict). We'll do the same.
(They should only be in the first line of a list item or a headline)
including org files is more complex - e.g. footnotes need to be namespaced to
their source file. org does this by prefixing each included files footnotes
with a number - but even that is not enough as it doesn't guarantee
uniqueness.
As I don't have a usecase for it, I'll avoid the additional complexity for
now.
... hopefully correctly
This will hopefully improve ease of developing (more granular test results) and
prepares us for adding example org->html renders for gh-pages
