[css-overflow-3] Forward-compat proofing

* Separate the triggering of the break (max-lines) from turning an
element into a discarding fragmentainer (continue), to be forward
compatible with other types of css-created fragmentainers, such as
continue:fragments or css-regions
* Make the line-clamp shorhand set the continue property as well
* Allow block-overflow to be set via the shorthand
* Various editorial tweaks

Closes w3c#2860

Author: frivoal