Cocoa: Initial implementation of glfwDragWindow

src/cocoa_window.m

diff --git a/src/cocoa_window.m b/src/cocoa_window.m
index c7f15ff..9de5837 100644
--- a/src/cocoa_window.m
+++ b/src/cocoa_window.m
@@ -1300,6 +1300,7 @@
 
 void _glfwPlatformDragWindow(_GLFWwindow* window)
 {
+    [window->ns.object performWindowDragWithEvent:[NSApp currentEvent]];
 }
 
 void _glfwPlatformSetWindowMonitor(_GLFWwindow* window,