
A notification can have two ways to specify a sound, via a name of a sound in the sound theme and/or a file. Therefore introduce an object that can hold both properties and creates an abstraction over the different source. This reduces the number of properties for a notification, which will make it simpler to port it to GObject properties. Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3173>