Add regression test

Add regression test

Fix issue 1290

Add optional horizontal-padding argument to Skyline::distance call
If horizontal padding is non-zero, the padding will be added
for making the distance comparison.  Padding is added during
System comparison, so Systems can be farther apart than staves
within systems.

Add default value of skyline-horizontal-padding to the System grob.

Add regression test for this behavior, which includes an override.