Add support for chatting directly from IM notifications

Also reorganizes the notification layout to use an StScrollView; very
tall notifications are now scrolled instead of just taking up more and
more of the screen.

https://bugzilla.gnome.org/show_bug.cgi?id=608999

js/misc/telepathy.js

@@ -215,6 +215,10 @@ const ChannelTextIface = {
         { name: 'AcknowledgePendingMessages',
           inSignature: 'au',
           outSignature: ''
+        },
+        { name: 'Send',
+          inSignature: 'us',
+          outSignature: ''
         }
     ],
     signals: [