detect special attachment situations in the model

[ichthyo]

The handling of timelines and sequences, as designed now, requires to detect when a specific model configuration is created: attaching a BindingMO to the model root should automatically trigger creation of an asset::Timeline, while attachment of a Track creates an asset::Sequence. To implement this, we need a hook within PlacementIndex.

We need this to keep the model automatically normalised, but we don't need it for the first integration round, as there are (at least) 2 other possibilities to set up a timeline and sequence (using the default and creating them by explicit ctor call)