notifications: Support `sound` capability
This allows clients like telegram to work better as we can honor the feedback level.
See https://github.com/telegramdesktop/tdesktop/issues/28895
Needs https://source.puri.sm/Librem5/feedbackd/-/merge_requests/150 (but builds without it)
Simple test: notify-send "Sound test" -h STRING:sound-file:/path/to/sound.ogg
This (together with the feedbackd bits) lays also a bit of foundation for the sound bits in the v2 of the xdg-desktop-portal notification spec.
Edited by Guido Günther