The current way to subscribe to '4a-alsacore' events does not provide the possibility to specify to which event we want to subscribe, it only allows to provide audio card related events.

Changes need to be made in '4a-alsacore' to allow to select the event we want to subscribe.

This implies breaking '4a-alsacore' 'subscribe' verb compatibility with previous version of '4a-alsacore'. But, currently, the only other binding using '4a-alsacore' events is '4a-hal', and '4a-hal' modifications to support new '4a-alsacore' subscription will be pushed with '4a-alsacore' modifications.

Also clean some pieces of code in relation with this change.