fine tuning of tie formatting

[Werner LEMBERG]

Have a look at the attached image.  I suggest to slightly modify the
algorithm of LilyPond so that it follows this additional rule.

  . Within a chord (this is, notes which are neither the topmost nor
    the bottommost note), the length of a tie must be smaller than the
    ties for the outermost notes if the distance to the next note (in
    the tie's direction) is smaller than a fifth (or even a sixth).
    As can be seen in the image, a fourth is too small: The tie almost
    touches the note heads.


      Werner