New alert and notification sounds

GNOME's existing set of alert sounds has been with us for a very long time, and could do with a refresh. We now also have the capacity to have a configurable notification sound, but are lacking sounds to go with it.

What we need:

• 4 or 5 new "alert" sounds (with one picked as the default)
• 4 or 5 new notification sounds (also with one picked as the default)

The alert sound is typically played to indicate keyboard input errors, like when someone presses backspace and there's no character there to be deleted. The notification sound can be played when an app shows a notification popup - think "you got a message".

Each sound is provided as a short OGG file. The existing ones can be seen here, and are about 0.75 seconds long.

We also might want to think about sounds for alarms and even ringtones, although that is probably out of scope for this initial exercise.