app: don't leak all cached GtkTreeIters in container views
This is evil: the hash table of "insert_data" is created on demand when GimpContainerView API is used, using a value_free_func that is set in the interface_init functions of its implementors. Therefore, no GimpContainerView API must be called from any init() function, because the interface_init() function of a subclass that sets the right value_free_func might not have been called yet, leaving the insert_data hash table without memory management. Call GimpContainerView API from GObject::constructed() instead, which runs after everything is set up correctly.
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