title: kendo grid dynamic field-editable definition
body: <p>I have kendo-ui grid, with some fields.
  I need one of the fields to be editable on add new row, and not editable on update row.</p>
  
  <p>I try to change data-source definitions before add row, and change it back before update.</p>
  
  <p>But the changing doesn't help.</p>
  
  <p>Is there any way to do it?</p>
  
  <p>Here is what I tried to do:</p>
  
  <pre><code> var schema = {
                  data: 'results',
                  model: {
                      id: 'GroupCode',
                      fields: {
                          GCode: { editable: false },
                          GroupPrincipalId: { editable: false },
                          GroupPrincipalName: { editable: false },
                          ChildCount: { editable: true },                          
                      }
                  }
              };
  
  onAddClick: function(){
    var gridElement = ('#myGrid').data('kendoGrid');
        gridElement.dataSource.options.schema.model.fields.GroupPrincipalId.editable = true;
        gridElement.dataSource.options.schema.model.fields.GroupPrincipalName.editable = true;
  
        gridElement.addRow();
  }
  </code></pre>
  
  <p>(onAddClick is called by my custom adding-button, not related to kendo-adding-logic);</p>