Add syntax for debugging slurs (showing & tweak control points)

[noteflakes]

Show control points:

8a (?6bc de8d)

Tweak control points:

8a (!{[0,0.5][0,0.5][0,0.5][0,0.5]}6bc de8d)

// omit zero values:
8a (!{[,0.5][,0.5][,0.5][,0.5]}6bc de8d)

// move all control points by same displacement:
8a (!{0,0.5}6bc de8d)

// change only second segment of broken slur ({} means 0 displacement)
8a (!{}{0,0.5}6bc \break de8d)

Things to consider:

• Tweaking ties:

//show control points
1c~? 8c
//tweak:
1c~!{[0,0.5][0,0.5][0,0.5][0,0.5]} 8c 

• mode-specific tweaks

Documentation for modifying slurs:

http://lilypond.org/doc/v2.18/Documentation/notation/modifying-shapes

Show control points code here:

https://github.com/openlilylib/snippets/blob/master/debugging-layout/display-control-points/display-curve-control-points.ly