[canberra] Fix memory leak and optimize silent playback.

If event has sound.volume property with value 0, don't really play the
sample but call complete immediately.

Also previously canberra plugin called n_sink_interface_complete() from
canberra_sink_play() where the request is not yet in playing state and
calling complete doesn't do anything. Added completion callback which is
called after plugin is in playing state.
Merge pull request #29 from jusa/dev

Fix memory leak and optimize silent playback in canberra plugin