Avoid some caret ghosts

Avoid some caret ghosts

When the caret is at end of row, if may happen that it is drawn after
the end of the row. In this case caret blinking will not work
properly. This patch extends the row background on the left and right
by Inset::TEXT_TO_INSET_OFFSET. This is only a hack that will not work
if the caret has a ridiculous width like 6.

Additionally, introduce some (disabled) debug code that numbers the
rows on screen by painting order.

Finally, make the code that detects whether the caret was in a given
row more precise (take boundary into account).

Fixes (mostly, see above) bug #10797.