Making a few corrections to the serialization attributes after having tested ContentType Serialization.
Enabling bulk saving of new ContentTypes with Parent/Composition dependencies.
Adding PropertyGroup and PropertyGroupCollection classes U4-926.
Adding PropertyType and PropertyTypeCollection classes U4-925.
Adding DataTypeDefinition and DataTypeDatabaseType U4-927.
Had to update the Property ref. on page and DynamicDocumentExtensions because of a conflict with the new Property class.
