Avoid row breaking at inconvenient places.

Avoid row breaking at inconvenient places.

When it turns out that breaking a STRING row element was not
sufficient in Row::shortenIfNeeded, we still remember the shortest
width that one can obtain. Later, when we try to split a previous
element of the row, we have a better idea of how much of the row
remains after it.

To this end, change the signature of Element::splitAt to use an enum:
FIT (was: force=false), FORCE (was: force= true) and BEST_EFFORT
(split at max_width, but do not return an error if the string is too
large).

Fixes bug #12660.