Deprecated public use of DocumentPart

A DocumentPart should be an internal aspect of the DocumentModel. But it's currently visible to outside code through DocumentModel methods.

With this change, the DocumentPart class and the DocumentModel.getPart(schema) and DocumentModel.getParts() methods are now deprecated. Two new additional methods are provided to achieve the same goals: DocumentModel.getPropertyObject(schema, name) and DocumentModel.getPropertyObjects(schema).