[css-break][css-text] Extending break-* to lineboxes

The break-* properties would be pretty useful to control inline layout.
We might want to defer that to the next level (i.e. Text/Fragmentation 4),
but it's something I think we should consider. Alternately, we'd want
some other way to suppress (but not forbid) line breaking within an
inline, since some form of keep-together is very useful when formatting
headlines and inlined metadata.

~fantasai