Fix a bunch of problems with handling sound/notifications related options
coming from the backend:
- `volume` and `notificationSoundsEnabled` live in a different settings
object; adjust the stores for that
- unbreak loading the actual wav/mp3 files (was never working on Linux
outside of AppImage!) -> use "qrc:/", same as the NIM backend does
- fix the volume scaling (backend has [1,100], QML is [0, 1])
- simplify our Audio QML item interface; no need for an extra property
and resolving the URL twice (which results in broken scenarios mentioned
above); we know which sounds we want to load
Closes#8668
Lukáš Tinkl
Patryk Osmaczko
Alexandra Betouni
Sale Djenic
¨Anthony