Make sure that rows are repainted when they get (un)selected

Make sure that rows are repainted when they get (un)selected

The bug is the following: when selecting several paragraphs quickly
enough, some rows do not get selected.This is a consequence of the
removal of row crc, which lead to not taking into account the
selection status of the row in the decision to repaint.

The solution chosen here is to add a Row::change() helper function to
modify row members. This will set the Row changed status whenever the
value of the member changes.