title: how to add a custom summaryitem to devexpress gridcontrol
body: <p>I have a summary field in footer of a gridcontrol. In the gridcontrol, I have CheckButtons on the first column for users to select the records to process on. I need to fix summary field only to sum selected rows. Now it sums every row. How can I get it sum only the selected rows?</p>
  
  <p><img src="https://i.stack.imgur.com/6nw7B.png" alt="enter image description here"></p>