Play a system sound when the volume is changed (Issue #4)

Every time the volume is changed in the plugin, the audio-volume-change event is emitted with libcanberra. This includes both with the media keys and the dialog. The feature is enabled with a checkbox option in Properties. The configure script now includes a check for libcanberra and options to enable/disable. There are no translations provided for the checkbox property text as I am monolingual. Tested with and without libcanberra support enabled.