define the model root

[ichthyo]

Define what to use as root of the high-level model. It should be a meta-object which can be created inline by the Index. Later on this MObject is expected to handle some kind of binding to session-global default handling rules. Probably it will give Advice to other parts of the model. Thus it might link to a suitable new kind of asset.

[ichthyo]

the model root could be either a one-off meta object, or it could be a Binding..?

[ichthyo]

...thus, following the reasoning regarding the Binding implementation, model root becomes a separate MObject subclass for now.

[ichthyo]

...and this new model root MObjec class isn't just a decoy; rather, by refactoring the way how PlacementIndex gets created, we can wire this root object magically back to the session, allowing to locate the rules and defaults store here. This opens a nice opportunity to get a grip at the assets lifecycle: now it's sufficient just to store/load them through the session root element

[ichthyo]

Implemented as a placeholder MObject for now. Can be created via the MobjectFactory. This happens when the SessionImpl object is created. Handling of the model root element is part of the PlacementIndex implementation, which ensures there is always an root scope.

Whhew. This one was by far more expensive than expected.

[ichthyo]

(In #282) * hash-ID == placement-ID == object instance identity == key within PlacementIndex storage table.

• a special SessionServices API was created, allowing PlacementRef to access the current session transparently.
• definition can be considered final (and documented thus far)