Changed names of derived classes of AudioProperties

Currently, the term "properties" is used for two things in taglib2 branch: tag contents and audio properties. It's a little confusing so I changed the names of the classes derived from AudioProperties from "Properties" to "AudioProperties".
But I think it's better to give a distinctive name to either of them. For example, change ```PropertyMap File::properties();``` into ```TagContentsMap File::tagContents();```