Move focus listeners to their own file

// FREEBIE

background.html

@@ -716,6 +716,7 @@
   <script type='text/javascript' src='js/signal_protocol_store.js'></script>
   <script type='text/javascript' src='js/libtextsecure.js'></script>
 
+  <script type='text/javascript' src='js/focus_listener.js'></script>
   <script type='text/javascript' src='js/notifications.js'></script>
   <script type='text/javascript' src='js/delivery_receipts.js'></script>
   <script type='text/javascript' src='js/read_receipts.js'></script>

js/focus_listener.js

@@ -0,0 +1,16 @@
+(function () {
+  'use strict';
+
+  var windowFocused;
+  window.addEventListener('blur', function() {
+    windowFocused = false;
+  });
+  window.addEventListener('focus', function() {
+    windowFocused = true;
+  });
+
+  window.isFocused = function() {
+    return windowFocused;
+  };
+
+})();

js/panel_controller.js

@@ -9,24 +9,12 @@
 
     window.Whisper = window.Whisper || {};
 
-
-    var inboxFocused = false;
-    window.addEventListener('blur', function() {
-        inboxFocused = false;
-    });
-    window.addEventListener('focus', function() {
-        inboxFocused = true;
-        clearAttention();
-    });
-    window.isFocused = function() {
-        return inboxFocused;
-    };
     window.isOpen = function() {
         return inboxOpened;
     };
 
     window.drawAttention = function() {
-        if (inboxOpened && !inboxFocused) {
+        if (inboxOpened && !isFocused()) {
             if (window.keepClear) {
                 clearInterval(window.keepClear);
                 delete window.keepClear;