Let getPosNearX take horizontal scrolling into account

Let getPosNearX take horizontal scrolling into account

If we do not do that, it is not possible to position the cursor after
a long inset with the mouse.

To do this, it is necessary to add the pit information to the Row
object. This is a good idea in any case, and will allow to simplify
some code later on.

Fixes bug #10094.
(cherry picked from commit 8851645799ef67015e49fd75b9dfeed65d685e85)