[MessageTray] Clean up source-vs-notification icon handling

A Source needs exactly one summary icon (which in the case of a
trayicon-based source won't even be just an image), but possibly many
notification icons, which may vary for successive notifications
(particularly in the case of NotificationDaemon notifications). So
differentiate these cases in the API.

https://bugzilla.gnome.org/show_bug.cgi?id=627303

js/ui/telepathyClient.js

@@ -475,10 +475,13 @@ Source.prototype = {
         this._receivedId = this._channelText.connect('Received', Lang.bind(this, this._messageReceived));
 
         this._channelText.ListPendingMessagesRemote(false, Lang.bind(this, this._gotPendingMessages));
+
+        this._setSummaryIcon(this.createNotificationIcon());
     },
 
-    createIcon: function(size) {
-        return contactManager.createAvatar(this._conn, this._targetHandle, size);
+    createNotificationIcon: function() {
+        return contactManager.createAvatar(this._conn, this._targetHandle,
+                                           this.ICON_SIZE);
     },
 
     clicked: function() {