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 be
count positions to the right of the old cursor
count is negative then the new strong cursor position will be
count positions to the left of the old
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.
number of characters to move (negative moves left, positive moves right)