Move the iterator a given number of characters visually, treating it as the strong cursor position.
count is positive, then the new strong cursor position will
count positions to the right of the old cursor position.
count is negative then the new strong cursor position will
count positions to the left of the old cursor position.
In the presence of bi-directional text, the correspondence between logical and visual order will depend on the direction of the current run, and there may be jumps when the cursor is moved off of the end of a run.
The data is owned by the caller of the method.
Number of characters to move (negative moves left, positive moves right)