title: Enable multiple selection of rows in NStableview
body: <p>I created a small application that is based on the structure of <a href="https://developer.apple.com/cocoa/coredatatutorial/index.html" rel="nofollow">this example</a> application.
  But now I need you to select multiple rows in tableview, I tried to find a solution but did not succeed. Could you tell me how should I do? Keep in mind that I am no expert. Thank you in advance.</p>