Cocoa: Remove subclassing of NSApplication

This removes the GLFW NSApplication subclass as a step towards better
coexistence with other libraries that touch Cocoa.

This moves application object creation to platform init to allow event
processing before window creation.

Related to #1317.

src/cocoa_platform.h

@@ -109,7 +109,9 @@ typedef struct _GLFWlibraryNS
     TISInputSourceRef   inputSource;
     IOHIDManagerRef     hidManager;
     id                  unicodeData;
-    id                  listener;
+    id                  helper;
+    id                  keyUpMonitor;
+    id                  nibObjects;
 
     char                keyName[64];
     short int           keycodes[256];