GNotification: Add support for custom sound provided by the theme
Submitted by Corentin Noël
Here is a branch that implements it: https://gitlab.gnome.org/GNOME/glib/commits/wip/tintou/gnotification-sound-theme
Bug #788802 was marked as won't fix but it's clear that playing the sound in the app itself is a no-go as the goal here is to toggle on/off the notification sound in the notification settings panel.
As many other things in the GNotification object, it is not required that the notification server will support this setting, nor that it will respect it in any way.