kcms/notifications: Always allow sound preview

Original commit message

kcms/notifications: Always allow sound preview

Reason for the change

Previously disabling the Sound action for a notification would also disable the preview button. This change allows playback at all times when there is a sound selected, such that users can check quickly what the sound would be, even if the notification sound is currently disabled.

Previous additional changes (enabling sound selection as well, and disabling the Sound action when no sound is selected) have been removed, see discussion.

Test plan

The actual sound playback, as well as the Reset and Defaults buttons at the bottom of the KCM are currently broken for me, but those are separate issues.

The relevant build target is kcm_notifications

Screenshots

Before	After