kcms/soundtheme: Add setting to disable notification sounds

Add a new setting option to control whether the notification sounds are globally enabled. This affects application and services that use KF6 KNotification.

It uses a checkbox instead of a switch because the setting is not instant-apply.

General task: #84 (closed)
Requires frameworks/knotifications!120 (merged) to be effective, and kde-gtk-config!89 (merged) for synchronization with the gtk apps

CCing @nclarius @broulik and @sitter, who showed interest in this feature