Simplify openConversation

Now that the InboxView is initialized in the background page context, we
can manipulate it more directly, without going through a global function
on the foreground window.

// FREEBIE

js/panel_controller.js

@@ -97,7 +97,7 @@
     var open;
     window.openConversation = function(conversation) {
         if (inboxOpened === true) {
-            appWindow.contentWindow.openConversation(conversation);
+            owsDesktopApp.openConversation(conversation);
         } else {
             open = conversation;
         }