Add removeEventListener() for the onload function

[micz]

Please note the following for the next update:

1. If the addEventListener() is used once (e.g. for 'load'), add-on should removeEventListener() as soon as its function is called and free the resources.
   elem.addEventListener(event, myfunction, false);
   function myfunction() {
   // not needed any more
   elem.removeEventListener(event, myfunction, false);
   // rest of the function code
   }