Reading the property has a few warts (see FIXMEs in the included
tests), but with this the timestamps on http://45.33.8.238/
get localized :^)
Since the Date() constructor currently ignores all arguments,
they don't get localized correctly but are all set to the current
time, but hey, it's still progress from a certain point of view.
This requires moving remove_all_children() from ParentNode to
Node, which makes ParentNode.cpp empty, so remove it.
It also co-opts the existing Node::text_content() method and
tweaks it slightly to fit the semantics of Node.textContent.
