Double references to plug-ins and instances

[mottosso]

Goal

Optimise memory layout and reference count.

Motivation

At the moment, instances are stored twice; once in the controller and once in the model.

• model.py
• control.py

The same goes for plug-ins.

This is dirty makes it unclear which of the two paths to take for access to these objects.

Implementation

Store both the Context and plug-ins in the corresponding model, and access it only from there. This includes the control.py:_iterator() which will need some minor refactoring to work that way.