Cocoa: Implement window cascading

Related to #195.

src/cocoa_platform.h

@@ -113,6 +113,7 @@ typedef struct _GLFWlibraryNS
     short int           keycodes[256];
     short int           scancodes[GLFW_KEY_LAST + 1];
     char*               clipboardString;
+    CGPoint             cascadePoint;
     // Where to place the cursor when re-enabled
     double              restoreCursorPosX, restoreCursorPosY;
     // The window whose disabled cursor mode is active

src/cocoa_window.m

@@ -1025,6 +1025,7 @@ static GLFWbool createNativeWindow(_GLFWwindow* window,
     else
     {
         [window->ns.object center];
+        _glfw.ns.cascadePoint = [window->ns.object cascadeTopLeftFromPoint:_glfw.ns.cascadePoint];
 
         if (wndconfig->resizable)
             [window->ns.object setCollectionBehavior:NSWindowCollectionBehaviorFullScreenPrimary];