In addition to being reference-counted, all nodes that are part of a
document must also keep the document alive.
This is achieved by adding a second ref-count to the Document object
and incrementing/decrementing it whenever a node is created/destroyed
in that document.
This brings us much closer to a proper DOM lifetime model, although
the JS bindings still need more work.
We use this to ensure that we're always working with a selection where
the start() is before the end() in document order. That simplifies all
the logic around this.
Scripts loaded in this way will block the parser until they finish
executing. This means that they see the DOM before the whole document
has been fully parsed. This is all normal, of course.
To make this work, I changed the way we notify DOM nodes about tree
insertion. The inserted_into() callbacks are now incrementally invoked
during parse, as each node is appended to its parent.
To accomodate inline scripts and inline style sheets, we now also have
a children_changed() callback which is invoked on any parent when it
has children added/removed.
