Rework caret display code

Rework caret display code

The new code is much simpler: what it does is, after redrawing has
been done, to mark the cursor row as changed, so that it will be
repainted on next paint event.

This avoids some crashes at the price of possibly repainting the row
when it was not necessary.

(cherry picked from commit 2f1eb35b868aaa5adb83ff358c2468551e25685e)