`ConstraintSet.clone` does not clone custom constraints

[bishiboosh]

When using ConstraintSet.clone to clone an existing ConstraintSet, its properties are cloned, but custom constraints aren't, whereas other "copy" methods like readFallback do copy custom attributes. Is there a reason for that ?

[bishiboosh]

(if this is indeed a bug, I can totally make a PR and add the needed code to clone the custom constraints)