Refactor usage of deprecated labelling of boolean radiobutton

Working on HIPPLUG-1573, I learned that the deprecated way of supplying labels for a boolean radio button field by means of "trueLabel" and "falseLabel" editor configuration properties, rather than through a regular value list, is still in use by the Channel Manager's content service (and maybe other places in the product?). Before the selections plugin can be cleaned up, this part of the Channel Manager needs to be refactored, which, in itself, represents a BC breaking change.