policy: pass private pointer as signal user-data for device handlers
We want to unregister the signals at cleanup time via g_signal_handlers_disconnect_by_data(). This saves us from storing the signal handler id or by naming the function explicitly via g_signal_handlers_disconnect_by_func(). However, the registered user-data @self is a public pointer. That is ugly, because potentially another component could register a signal with passing the public @self pointer as user-data. Although that doesn't currently happen, it is more correct to register with a private pointer to avoid this case altogether.
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