Consistency between const and non-const params for clone functions

Created by: EmielBruijntjes

The icalvalue_new_clone() function expects it's parameter to be const. This makes sense, because the to-be-cloned value is not going to be altered. All the other *_new_clone() functions in this library however, have a non-const parameter, even though they are not going to alter their argument either. This unnecessary inconsistency make it harder to create a common clone and destruct signature when you try to make a common C++ template class for all these types. This pull requests fixes the inconsistency.