Set the sound name as an EVENT_ID property to canberra, instead of the previous filename-based implementation.
The Url lookup to set the MEDIA_FILENAME is kept as a fallback for non-ported configs. The library will play this file if the sound name is not found in the theme.
This is a step towards including sound-themes support (see plasma/plasma-workspace#47 and raploz/blue-ocean-sound-theme#6), by allowing notification sounds to be specified by "event name" and not by filename. Due to the fallback, the previous notification sounds should still work properly.
The sound theme is read from
Theme, defaulting to our current de-facto theme ("oxygen").
I'm not sure about what to do with the Phonon path here.