keybindings: Add a key binding to toggle the microphone's mute status

Provided additional "mic-mute" key binding.

For reference: mate-desktop/mate-settings-daemon#253

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

Author: zhangxianwei8

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

@@ -13,6 +13,8 @@
 
 	<KeyListEntry name="volume-up-quiet" description="Volume up quietly" />
 
+	<KeyListEntry name="mic-mute" description="Microphone mute" />
+
 	<KeyListEntry name="media" description="Launch media player" />
 
 	<KeyListEntry name="play" description="Play (or play/pause)" />