file-menu:new-callback in frame:open-here-mixin

This method is called when the new menu-item of the file-menu menu is selected.

• (send a-frame:open-here-mixin file-menu:new-callback item evt void
     item : (instance (derived-from menu-item%))
     evt : (instance control-event%)

  When the preference 'framework:open-here? preference is set, this method prompts the user, asking if they would like to create a new frame, or just clear out this one. If they clear it out and the file hasn't been saved, they are asked about saving.