improve the custom attributes for embedded objects.

currently the [name_of_XML_tag] customizes AvailableClasses and CustomAttributes based on the class or content type of the embedded object, as defined here:
https://doc.ez.no/eZ-Publish/Technical-manual/4.x/Reference/Configuration-files/content.ini/name_of_XML_tag

but, if we add a custom attribute in content.ini for an Image for example, then any embedded image edited will display that attribute.

this features requests customized tags attributes restrictable to content classes. for example, we want a custom class/attribute for image that is only shown/editable when the embedded image object is being edited in a Article content.