Until now we would simply apply stylesheets in the order they finished
loading. This patch adds a StyleSheetList object that hangs off of each
Document and contains all the style sheets in document order.
There's still a lot of work to do for a proper cascade, but at least
this makes us consistently wrong every time. :^)
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.