Fix positioning of flows after being bumped to the next line

* fixes #626
* got rid off the whole position_modifier we dragged along - it
  was buggy for bumping to the next line. Because the position
  modifier from bumping to the next line was applied to the second
  element in that line/row. Didn't notice because next_line_start
  already took care of that bumping logic
* need to handle the special case that when being positioned in
  the same line we have to move the element one pixel to the
  right... but not for the first element :/
Add offset for next element when updating current_position

* niftier approach that I'm much happier with!
* also introduced constant to represent that concept - no more
  magic +1s :)
Update reference to now somewhat updated constant
Oops forgot to get rid code and comment from first approach
Merge pull request #726 from shoes/fix-off-by-one

Fix off by one error in slot positioning. Changelog: bugfix