Adding the PropertyEditor model as internal to slowly adopt it using a slightly different model approach then in v5.
Minor refactoring of IEntity/Entity.
Adding some notes and small change to IContentBase, PropertyValues and the ability to set property values with an anonymous object is shared for Content and Media.
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.
