Fix caret painting

Fix caret painting

The trick is to remember in BufferView what has been done at the
previous draw, so that the row that contained the caret can be
repainted if needed.

To this end, add an argument paint_caret to BufferView, although
painting the caret is not the job of the BufferView (at this point).

BufferView::needRepaint will act as an interface with
TextMetrics::drawParagraph to know whether the painting of a given
row should be forced.

Currently everything is done at the top row level, so that, if the
caret is in a large table, the whole table will have to be repainted.
It is not clear yet that this is necessary.