title: How to have multiple files in tabs in Xcode
body: <p>in the past (older version of xcode) I had a plus sign in the top right of an editor to add a tab. Then in the new tab I could click on the second file and I had 2 files open in 2 tabs.
  My plus sign is gone (I guess with my last update). </p>
  
  <p>How to rollback this + button or how to have tabbed editors in Xcode 6.4 (6E35b) ?</p>