Support ties/slurs

[MatthewDorner]

VexFlow supports this via Curve and StaveTie. I think Curve can be used for everything. The problem I've had so far is in the case where a tie/slur begins on the last note of the last measure of one line, and is meant to connect to a note at the beginning of the next line. If you tell VexFlow to do this like a normal tie/slur, it'll draw the curve across the entire page diagonally.

I've seen sheet music handle it like this:

But I can't figure out how to do this in VexFlow. I tried inserting a GhostNote and drawing the curve between the real note and the ghost note, but it gives an error. I think it was something like getTiePositionX doesn't exist.