keybindings: Add some key bindings to control volume quietly

Provided three new key bindings to volume control:
"volume-mute-quiet"
"volume-down-quiet"
"volume-up-quiet"

For reference: mate-desktop/mate-settings-daemon#252

Signed-off-by: Zhang Xianwei <[email protected]>

Author: zhangxianwei8

capplets/keybindings/00-multimedia-key.xml.in

@@ -7,6 +7,12 @@
 
 	<KeyListEntry name="volume-up" description="Volume up" />
 
+	<KeyListEntry name="volume-mute-quiet" description="Volume mute quietly" />
+
+	<KeyListEntry name="volume-down-quiet" description="Volume down quietly" />
+
+	<KeyListEntry name="volume-up-quiet" description="Volume up quietly" />
+
 	<KeyListEntry name="media" description="Launch media player" />
 
 	<KeyListEntry name="play" description="Play (or play/pause)" />