Use onClosed instead of onSuspend

Renames extension.windows.beforeUnload to onSuspend, to match the
underlying chrome api call. onClosed fires when the frontend app window
is closed, while onSuspend fires when the background page is closed or
refreshed (which amounts to an app restart).

Frontend views are initialized iff the inbox window is opened, and so
should always be listening to onClosed in order to know when they are no
longer needed.

// FREEBIE

js/panel_controller.js

@@ -63,8 +63,8 @@
                     inboxFocused = true;
                 });
 
-                // close the panel if background.html is refreshed
-                extension.windows.beforeUnload(function() {
+                // close the inbox if background.html is refreshed
+                extension.windows.onSuspend(function() {
                     // TODO: reattach after reload instead of closing.
                     extension.windows.remove(inboxWindowId);
                 });