In Segment.Split, we didn't take cell width into account
when calculating the offset, which resulted in some "fun" bugs.
I've added a new overload for Segment.Split and obsoleted the old one.
Closes#150
The bug might occur if there are wide characters such as emojis
at the end of a line. The SplitLines method mixed cell width
and text length, which might give incorrect results. This commit
makes sure that comparison and calculation is done using cell width
where it's appropriate.