Fix flushing of row that was cut after an hyphen

Fix flushing of row that was cut after an hyphen

When using Qt stuff in breakAt, it may happen that the row is broken
after an hyphen (whereas the old code would only consider spaces).

The fact that we abuse the Row::right_boundary() property to detect when
a row should be flushed broke justification when a row is cut at an
hyphen.

Fix this by introducing a new Row::flushed() property and set it as needed.
(cherry picked from commit 8e7d0c2002bdc69c95f3a43f7c78d13fe47ce5f3)