Issue 3999: Make tablature half-note stem-spacing adjustable.

This adds a new property to the Stem grob called 'double-stem-separation (default=0.5), that allows users to adjust the space between the double-stemmed half-notes in tablature:

\new TabStaff {
  \tabFullNotation
  c4 c2 c4
  \override Stem.double-stem-separation = 0.3
  c4 c2 c4
}

It also centers the stems on the fret number and adjusts the X-extent accordingly.

On the tracker:
http://code.google.com/p/lilypond/issues/detail?id=3999

[dak, 2014-07-09 07:02:48]

https://codereview.appspot.com/110960043/diff/1/scm/define-grob-properties.scm
File scm/define-grob-properties.scm (right):

https://codereview.appspot.com/110960043/diff/1/scm/define-grob-properties.sc...
scm/define-grob-properties.scm:243: stems of a half note in tablature when using
@code{\tabFullNotation},
@code{\\tabFullNotation}

https://codereview.appspot.com/110960043/diff/1/scm/define-grob-properties.sc...
scm/define-grob-properties.scm:245: of the default @code{Staff} staff-space
height.")
Why @code{Staff} here? Is this specific to Staff contexts only?  That would mean
that this is not in @code{TabStaff} ?

[Mark Polesky, 2014-07-09 07:13:04]

On 2014/07/09 07:02:48, dak wrote:
> https://codereview.appspot.com/110960043/diff/1/scm/define-grob-properties.scm
> File scm/define-grob-properties.scm (right):
> 
>
https://codereview.appspot.com/110960043/diff/1/scm/define-grob-properties.sc...
> scm/define-grob-properties.scm:243: stems of a half note in tablature when
using
> @code{\tabFullNotation},
> @code{\\tabFullNotation}

I always get confused with that.  Are there some places where @code{\foo} is
allowed and other places where it is not?

>
https://codereview.appspot.com/110960043/diff/1/scm/define-grob-properties.sc...
> scm/define-grob-properties.scm:245: of the default @code{Staff} staff-space
> height.")
> Why @code{Staff} here? Is this specific to Staff contexts only?  That would
mean
> that this is not in @code{TabStaff} ?

Hm.  How to word this?  If 'double-stem-separation were set to 1, the horizontal
distance between the stems would be the same as the vertical distance between
two staff-lines in a *regular* staff (i.e. the default
Staff.StaffSymbol.staff-space = 1).  But TabStaff is not a regular staff, since
its staff-lines are further apart (i.e. the default TabStaff.Symbol.staff-space
= 1.5).  If you have a better way to convey this, let me know.

Mark

[Mark Polesky, 2014-07-09 07:14:43]

On 2014/07/09 07:13:04, Mark Polesky wrote:
> ...But TabStaff is not a regular staff, since its staff-lines are
> further apart (i.e. the default TabStaff.Symbol.staff-space = 1.5).

Pardon the typo; I meant TabStaff.StaffSymbol.staff-space of course.

[dak, 2014-07-09 07:39:38]

On 2014/07/09 07:13:04, Mark Polesky wrote:
> On 2014/07/09 07:02:48, dak wrote:
> >
https://codereview.appspot.com/110960043/diff/1/scm/define-grob-properties.scm
> > File scm/define-grob-properties.scm (right):
> > 
> >
>
https://codereview.appspot.com/110960043/diff/1/scm/define-grob-properties.sc...
> > scm/define-grob-properties.scm:243: stems of a half note in tablature when
> using
> > @code{\tabFullNotation},
> > @code{\\tabFullNotation}
> 
> I always get confused with that.  Are there some places where @code{\foo} is
> allowed and other places where it is not?

Nothing confusing here.  In Texinfo, backslash is an ordinary character, so you
*always* write @code{\tabFullNotation} and similar in Texinfo.  But this Texinfo
section here is inside of a Scheme string, and backslashes in Scheme strings
need to be escaped.  Nothing to do with Texinfo.