beginReq and endReq should be public properties of grid

When using a custom defined ``datatype`` function for retrieving data, it's best to use the already existing methods for ``loadui``.

Here's an example:
```js
jQuery("#list").jqGrid({
  loadui: 'block',
  datatype: function(args){
    var grid= this;
    grid.beginReq(); // Show the loading indicator
    CustomAjax.invoke({
      url: 'hosting.php',
      params: args,
      successCallback: function(response) {
        grid.addJSONData(response.getValue('accounts'));
        grid.endReq(); // Hide the loading indicator
      },
      errorCallback: function(response) {
        // ... Add error to message container
        grid.endReq(); // Hide the loading indicator
      }
  }
  ...
});
```
Kind regards,
      g.