Keep inbox window open, but allow it to be hidden

As a chrome packaged app, we have to keep at least one window open in
order to maintain our websocket connection in the background page.

This change replaces the system window frame with custom buttons in the
inbox header, such that the 'close' button merely hides the window
rather than unloading it.

Fixes #237
FREEBIE

js/panel_controller.js

@@ -121,6 +121,7 @@
                 id: 'inbox',
                 url: 'index.html',
                 type: 'panel',
+                frame: 'none',
                 focused: true,
                 width: 260, // 280 for chat
                 height: 440 // 420 for chat