outside-staff-padding: add just one pad; issue 2910

When two grobs using 'outside-staff-padding are placed 
against each other, use the larger of the two grobs'
values of 'outside-staff-padding.
Same for 'outside-staff-horizon-padding.

Commit 28f32949 had started to place two pads between outside-staff items, but just one pad to the staff. The C-code-default for 'outside-staff-padding was also inconsistent between locations in the code.

Since then, some padding values were defined for individual grobs. These values are no longer needed.

[Janek Warchol, 2013-09-01 16:54:00]

Could you add some information in the commit message explaining why had you
changed the default values?

[Keith, 2013-09-02 20:10:36]

On 2013/09/01 16:54:00, Janek Warchol wrote:
> Could you add some information in the commit message explaining why had you
> changed the default values?

I didn't think anyone would care.  I added some sentences above.
We were trying to adjust padding values to remove problems caused by the
skyline-spacing, but had not yet realized that the padding was being
inconsistently applied by the new code.

[Trevor Daniels, 2013-09-04 22:27:18]

https://codereview.appspot.com/10329043/diff/20001/Documentation/notation/spa...
File Documentation/notation/spacing.itely (right):

https://codereview.appspot.com/10329043/diff/20001/Documentation/notation/spa...
Documentation/notation/spacing.itely:2726: previously-positioned grobs can be
controlled with
I don't think we should talk about "previously-positioned"
grobs now the padding is the larger of adjacent grobs.
Better to say: "The vertical padding between two adjacent
outside-staff objects is the larger of each object's
@code{outside-staff-padding}."

[Keith, 2013-09-04 23:49:02]

On 2013/09/04 22:27:18, Trevor Daniels wrote:

> I don't think we should talk about "previously-positioned"
> 
True, that.
"The vertical padding around outside-staff objects can be controlled with
@code{outside-staff-padding}."