Cache program preferences

So that qbt can just use the data from memory which is vastly faster than waiting for a response
over the net.

src/webui/www/private/scripts/client.js

@@ -1576,6 +1576,8 @@ window.addEvent('load', function() {
         }
     }).activate();
 
+    window.parent.qBittorrent.Cache.preferences.init();
+
     // fetch qbt version and store it locally
     new Request({
         url: 'api/v2/app/version',