Move the screenshot sound effects to gnome-shell

Re-proposing MR !3 (closed) by Brion Vibbor.

Companion patches:

• gnome-shell gnome-shell!1658 (merged)
• g-s-d gnome-settings-daemon!223 (merged)

Since it can take some time to compress a PNG image, at very high resolutions there was a noticeable delay before the audiovisual feedback on screenshots.

To keep from complicating the interface for the shell screenshot service, moving the sound effects into the service (which already did the visual effects) so it can trigger them after capture and before compression.

Note this removes the canberra dependency for gnome-screenshot.