This makes multiple levels of quote actually use different quotation
marks, instead of always the first available pair of them.
Each Layout::Node remembers what the quote-nesting level was before its
content was evaluated, so that we can re-use this number in
`apply_style()`. This is a bit hacky, since we end up converting the
`content` value into a string twice.
`StyleProperties::content()` now takes an initial quote-nesting level,
and returns the final level after that content.
