messageTray: Don't play sound when banners are disabled (cherrypick)
Merging it to have it in Ubuntu 20.04 (LTS)
It is cherry-pick of the original commit. @fmuellner mentioned that he doesn't mind it being merged to 3.36.
Original commit message
When support for notification sounds was added, it made some sense to keep the 'enable-sound' setting independent from 'show-banners'.
However that changed when 'show-banners' was rebranded in the UI as "Do Not Disturb", as sounds are at least as disturbing as the banners.
Edited by Vadym Kurachevskyi