make sure that AmbitusLine is visible for small ambits

make sure that AmbitusLine is visible for small ambits

Until now, the gap between ambitus heads and ambitus line
was constant, so either the gap had to be very small or
the line wasn't visible in case of smaller ambits (like
4th or 5th). With this patch ambitus gap will be calculated
dynamically, depending on the distance between ambitus heads.

Also, the default gap for bigger ambits is changed so that
ambitus line will end precisely in the staffline or in the
middle of staffspace.