add display-slur-control-points.ly
control pts: parametrize thickness and size, change comments
control points: change default values
control points: change the way how they are turned on
control points: add some metadata
control points: turn on for LaissezVibrerTie and RepeatTie
control points: add todo
control points: doesn't work with LaissezVibrerTies...
control points: change usage example
control points: add todo
control points: document
control points: add png output example