Don't fetch messages from the background

Only fetch them from a frontend view. If the conversation is not open,
we don't need to load the messages, and if we do load them, they will
render before we've done the initial contact info loading (as
implemented in 74e96ce).

Fixes #344

// FREEBIE

js/panel_controller.js

@@ -87,7 +87,7 @@
             conversation.fetch();
         }
         if (inboxOpened) {
-            conversation.fetchMessages();
+            conversation.trigger('newmessages');
             extension.windows.drawAttention(inboxWindowId);
         } else if (Whisper.Notifications.isEnabled()) {
             var sender = ConversationController.create({id: message.get('source')});