telepathyClient: Implement Sent signals.

This will allow us to update our notifications when
someone uses a different Telepathy client to send
messages.

https://bugzilla.gnome.org/show_bug.cgi?id=635991

js/misc/telepathy.js

@@ -272,7 +272,9 @@ const ChannelTextIface = {
     ],
     signals: [
         { name: 'Received',
-          inSignature: 'uuuuus' }
+          inSignature: 'uuuuus' },
+        { name: 'Sent',
+          inSignature: 'uus' }
     ]
 };
 let ChannelText = DBus.makeProxyClass(ChannelTextIface);