#7 object and association creation

Completely overhaul object creation and association creation.

In principle:

• object should be created separate from association creation;
• object ids should be independent from association instances;
• question: can non-default instances be entirely independent?

Obviously, this still needs to be worked out in detail.

Another feature: if a request to the object manager is made, a response is sent globally. If two requests are performed in the same step, however, those responses will arrive at the same time as well. We should be able to identify a request in some way.

Object manager interface (events):

• create_instance(class_name, constructor-parameters) -> instance_id
• associate_id(source_id, target_id) -> link_id
• delete_instance(instance_id)

The current start_instance should be merged with create_instance: instances should always be started when created.

Object manager interface (methods):

• get_type(instance_id) -> class
• get_instances(class)
• get_links(instance_id, association) -> link_ids