Add reasonable support for all values of white-space CSS property.
Values of the property are translated into a 3-tuple of rules:
do_collapse: whether whitespace is to be collapsed
do_wrap_lines: whether to wrap on word boundaries when
lines get too long
do_wrap_breaks: whether to wrap on linebreaks
The previously separate handling of per-line splitting and per-word
splitting have been unified. The Word structure is now a more
general Chunk, which represents different amounts of text depending
on splitting rules.
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