Change the way scrolled rows are tracked

Change the way scrolled rows are tracked

The goal of this commit is to simplify the logic in TextMetrics::draw.
Now, rows are repainted depending on their changed() status.

Instead of checking whether rows have been scrolled horizontally at
draw time, the code marks the row as changed when testing for
horizontal scrolling.

To this end a new method TestMetrics::setRowChanged is added, that
searches a row in the text metrics cache and marks it changed if
found.

The old code that remembered the previously scrolled row can now be
removed.