System Sounds KCM: Add an option to disable all notification sounds

Suggested by @nclarius in the discussion about Sound Theme KCM:

Not something that needs to be done in this in MR, but a thought; this would be a straightforward place to introduce an option to have no sounds for any system events.

(+1 comment) @iasensio

We currently have no other means to do this than muting the "notification sounds" stream on the Audio KCM, which is not ideal, so adding a new setting also seems in place.

We can sync with the gtk property Settings:gtk-enable-event-sounds via kde-gtk-config.