When a previous character needed to be found, it is simpler to move
existing cursor rather than retravese the linked list to find the offset
again.

Offsets are still used within the function, but this patch reduces the
use of it at the entry to the function.

The function was used in one place, and was simply a wrapper around a
call to ME_InsertRunAtCursor, so I removed it to avoids it use in other
parts of the code.

Previously the function was only used to move a single character in
either direction, so I made the function more general so that it could
be used in more places.