Integrate after-grace-terminated spanners

[trevorbaca]

\new Score <<
    \new Staff {
        \time 6/4
        \override DynamicLineSpanner.staff-padding = 5
        \set Score.tupletFullLength = ##t
        \set Score.proportionalNotationDuration = #(ly:make-moment 1 12)
        \times 3/2 {
            c'4
            \f
            \>
            #(define afterGraceFraction (cons 31 32))
            \afterGrace
            d'4
            {
                s4
                \p
            }
        }
        \times 3/2 {
            c'4
            \f
            \>
            #(define afterGraceFraction (cons 31 32))
            \afterGrace
            d'4
            {
                s4
                \p
            }
        }
    }
    \new Staff {
        \times 3/4 {
            c'4
            d'
            e'
            f'
        }
        \times 3/4 {
            c'4
            d'
            e'
            f'
        }
    }
>>

[trevorbaca]

Still valid in October 2018.

[trevorbaca]

Pros: won't malfunction at a line-break.

Cons: horizontal spacing won't ever be exact.

[trevorbaca]

Closing because fundamentally a hack.

Better to advocate for spanner improvements in LilyPond development.