is rebuilt when a content type's alias is changed or a property type is removed from it. Adds ability to rebuild
content xml structures for specified content types, not for all content. Adds ContentTypeExtensions and
more unit tests for ContentTypeTests. Changes RuntimeCacheProvider to use new ConcurrentHashSet instead of a dictionary
with the same key/value. Adds overload to IRepositoryCacheProvider to clear cache by type. Clears the IContent cache when
a Content type is saved.
All relates to fixing #U4-1943
