setattr_original_value should be a property of the `TraitType`

[mdickinson]

[From #1645]

TraitType.clone needs to know the value of setattr_original_value for the associated ctrait, resulting in this awkwardness in the clone method (following merge of #1645)

setattr_original_value = new.as_ctrait().setattr_original_value

Instead, we should make setattr_original_value an attribute of the TraitType itself.