title: Configure NSSplitView with three subviews like mail.app
body: <p>It is posible to configure splitview with 3 subviews through auto layout constraints like in mail.app? What i mean: when the user resizing the window only rightmost subview change it size, when the user drags the divider between the leftmost and middle subviews, only left and right subviews changes its size and when the user drags the divider between the middle and the right subviews only these subviews change size.
  If it is possible how to do the. 
  I think that this task may be performed by constraints priority, but i confused with that.
  Thanks!</p>