Use proper ref counting to free transaction_model's CustomList
The tree widget in the main window holds a reference to the transaction
model but (the horribly named) transaction_model_initialize() would free
up everything inside it.
Since CustomList _is_ a fully-fledged GObject, using its reference count
seems like an obvious solution to invalid memory errors (caught using
(cherry picked from commit e56f694f756e618388ed8fb4b863f03ad4e63e8f)