Change CharacterProperties.rtl to NestedBool
Created originally on Bitbucket by matthewonorman (Matthew Norman)
Was already merged in Bitbucket before import, marked as merged by the import user
This fixes a small error under which attempting to write out something with CharacterProperties using to_tree() causes an error because the rtl property is a Bool. Fixed by substituting it with a NestedBool. This is the second PR for this subject, pulled out of the 2.3 branch instead of default.