Vexflow needs a slur/phrase/tie curve type

[AaronDavidNewman]

Does it?

Slurs are currently implemented with curve.ts. While the implementation of curve is fine, the interface is limited and I don't know if that can be fixed in a backwards-compatible way. Instead, I propose having static functions that fill in slur parameters to make them equivalent to the existing curve or tie implementation.

The slur class would overcome the following limitations:

• allow an arbitrary number of control points
• allow explicit orientation up/down or auto
• allow placement on the note head side with a specific pitch index
• allow x-offset on begin and end sides of the slur
• line properties like dashed (maybe this can be done now with context?)