Control event volume

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

It would be nice if we could control event (alarm) sounds volume separately. This is useful when you have some video that itself has a low volume so you turn up the system volume but in return you also get very loud event sounds.

Start in the elementary/settings-sound repository by locating the existing event or alarm sound volume handling and its Switchboard settings entry points. Determine how a separate event-volume control would fit the current design; the work is done when event sounds can be adjusted independently from system or media volume.

Written by the indexing model from the issue text.