Draft: Support the xdg sound spec via libcanberra

This is a similar change to knotifications!105 (merged) because the code is mostly duplicated between the two repos.

Draft MR for for testing purposes of the "oxygen" and the new "blue-ocean" sound themes, and a step towards implementing proposal plasma/plasma-workspace#47 (comment 713539)

The theme's name is hardcoded to "oxygen", since it is already gained compatibility with the sound them spec (plasma/oxygen-sounds!6 (merged)).