Replaced remained RefCounter class with smart pointers and removed it

This removed ```RefCounter``` and ```RefCounterOld``` class as well.
From this time on, TagLib leaves all the atomic operations to the standard library when ```shared_ptr``` is available.

This also replaced ```List<T>::setAutoDelete()``` functionality with manual ```delete```s temporarily. I will replace them again with smart pointers in separate pull requests because the changes will be too large for a single pull request.