Fix off by one error in slot positioning

(short intro: we need position modifiers/offsets because if we have to let the element right of another element start one pixel next to it, i.e. absolute_right + 1, otherwise we'd have overlapping elements)

There is a positioning error described in #626 - basically the old approach to position modifiers was wrong for bumping to the next line. So the position modifier for bumping to the next line would wrongly be added to the SECOND element of the next line not the first. We didn't notice it because the first element had the correct position, as `current_position.next_line_start` already took care of the + 1 bumping down.

The code is much simpler now imo as we got rid off the whole `position_modifier` that always was a bit awkward. Now it is always in `update_current_position` - if we bump to the next line the code doing that positioning resets the x value either way. That also works around a nifty problem where we basically always want to add a +1 to the x coordinate if we position in the current_line except when it's the very first element...

Imo this is ready to go, would love a review and maybe a short look of @jasonrclark to see if it somehow impacts the TextBlock positioning!