Add support for new attributes in content objects

[dunkelstern]

Container Contents:

• new Attribute subtype can be one of
  • generic: no content definition, no index
  • list: all outlets of same type and named <container_name>_item, items contain index attribute
  • structblock: all outlet names are unique, no index
  • streamblock: outlets are not unique but contain index attributes

All Content types:

• Optional contains index attribute to mark item as part of a list

[p-fischer]

@tscholze @mredlin On Android, if the contents field in a Page only consists of one element, which is a ContainerContent, the hierarchy is flattened, i.e. the ContainerContent is kicked out and its children are assigned to Page's contents directly.

This together with the "startsWith" check, described in the comment of the Android issue
anfema/ion-client-android#173 (comment)
could enable that the BCG app+ION client works with both the old and new ION API

[mredlin]

@p-fischer
@tscholze mentioned that this is already used in the bcg apps 😅 .
Is there something to do for us concerning this issue or can we close it?

[p-fischer]

There are 2 changes to the ION models:

• In ContainerContent: new attribute subtype (see Jos description)
• In Content: new optional attribute index (and remove attribute position if that exists)

Neither subtype nor index were necessary in BCGyou to parse the data but you still might want to update the ION models

[mredlin]

@p-fischer Thanks for the explanation 👍