Allow \shape to tweak music, swap its arguments (issue 6585052)

[david . nalesnik]

LGTM


https://codereview.appspot.com/6585052/diff/1/input/regression/shape-other-curves.ly
File input/regression/shape-other-curves.ly (right):

https://codereview.appspot.com/6585052/diff/1/input/regression/shape-other-curves.ly#newcode18
input/regression/shape-other-curves.ly:18: \override PhrasingSlur
#'color = #blue
I wonder if it would be helpful to alter one or two of the following
applications of the function as override to the tweak form.

https://codereview.appspot.com/6585052/diff/1/input/regression/shape-slurs.ly
File input/regression/shape-slurs.ly (right):

https://codereview.appspot.com/6585052/diff/1/input/regression/shape-slurs.ly#newcode19
input/regression/shape-slurs.ly:19: g,8 f' e d c2)
What about adding a demonstration here of the ability to modify curves
beginning at the same timestep?  So possibly insert here something like
the following (paired with the alteration I've added below):

\break
\acciaccatura b8^( cis4 cis' fis, gis)

https://codereview.appspot.com/6585052/diff/1/input/regression/shape-slurs.ly#newcode33
input/regression/shape-slurs.ly:33: g,8 f' e d c2)
\break
\shape #'((0 . 0) (-0.1 . -0.25) (0.1 . -0.25) (0 . -0.25)) Slur
  \acciaccatura b8
    -\shape #'((-0.5 . 0.5) (0 . 0.5) (0 . 0) (0 . 0)) ^( cis4 cis' fis,
gis)

https://codereview.appspot.com/6585052/