title: How do I convert between global and local coordinates when working with NSScreens?
body: <p>I am writing an app that maniuplates window positions and am adding multi-monitor support.</p>
  
  <p>To facilitate this, I have the "local" window position (relative to the frame of an NSScreen) and I need to translate this to the "global" window position (relative to the position from which all monitor positions are designated, which I believe is the lower-left corner of the first (0th) index of [NSScreen screens]).</p>
  
  <p>I would like to use a single coordinate system to store my window positions and do this relative to the global coordinate system, then translate to the local coordinate system for display.</p>
  
  <p>What are the helper methods to translate coordinates between an NSScreen frame and the global screen-space?</p>