Play a sound when there are no idle entities.
If there are no idle units and the idle-unit hotkey is pressed there is now a sound played.
Beneficial when there are already units selected since selection doesn't change.
Sound courtesy of @Samualis.
Refs. #5360
Based on a patch by: @cpc
Differential revision: https://code.wildfiregames.com/D1692
Comments by: @asterix, @smiley, @Stan, @vladislavbelov