Re-use walkDOMDepthFirst we already had for turning the editor into
a string, which also allows us to detect when leaving a node better,
which is needed to decide how block elements create newline parts
is more centralized and robust manner, more in line with
how html works (consecutive block elements break the line)
instead of hacky special casing from before.
Also, to support every line of MD generated
within a BLOCKQUOTE, the parts an element creates are returned
and then centrally prepended with "> ", instead of passing in
the parts array and doing it in every parse... function.
