AbstractWindow.cleanup creates cyclic garbage.

[mdickinson]

The cleanup method of AbstractWindow introduces new reference cycles. Ironically, it was apparently introduced in commit 8e3aa33 in order to break cycles.

The problem lies in the line self.component = None here: https://github.com/enthought/enable/blob/master/enable/abstract_window.py#L385. That assignment triggers the _component_changed static Traits listener, which in turn creates a new component and assigns that to self.component. That then triggers another call to _component_changed which handily inserts a reference back to the window from the component, creating a cycle.