startup: move heavy loading out of DOM/HTML

app_main/app.htm
 * wrap heavy loading in setTimeout() instead of anon funciton
   (seen on <http://mathiasbynens.be/notes/settimeout-onload>)
 * a bit of postmature optimization

app_main/app_front.js
 * since there is no `onload` event after above change,
   use setInterval() to catch `Ext`