properties-window: Stop leaking extension models

We create a GListStore whose first reference we never release, so
it leaks and, with it, all objects which it owns a reference to.

We keep a GList for cleanup which gives the false illusion that we
are doing a good job at memory management, which is a lie.

Deeply free the list after use instead, and release the first
reference to the list store too. The list box keeps the list store
alive as its internal module and releases it when destroyed.