Cocoa: Hide cursor instead of using blank image

When cursor isn't in normal mode and should be hidden, use [NSCursor hide]
method instead of setting it to blank image. This should prevent
situations when hidden cursor becomes visible after system notification
was shown.

Fixes #971.
Closes #1028.

src/cocoa_platform.h

@@ -102,7 +102,6 @@ typedef struct _GLFWlibraryNS
     CGEventSourceRef    eventSource;
     id                  delegate;
     id                  autoreleasePool;
-    id                  cursor;
     TISInputSourceRef   inputSource;
     IOHIDManagerRef     hidManager;
     id                  unicodeData;