Updated SDL_syswm.h for SDL 3.0

* The header is no longer dependent on SDL build configuration
* The structures are versioned separately from the rest of SDL
* SDL_GetWindowWMInfo() now returns a standard result code and is passed the version expected by the application
* Updated WhatsNew.txt and docs/README-migration.md with the first API changes in SDL 3.0

test/testnative.h

@@ -15,9 +15,6 @@
 */
 #include "SDL.h"
 
-/* This header includes all the necessary system headers for native windows */
-#include "SDL_syswm.h"
-
 typedef struct
 {
     const char *tag;
@@ -36,10 +33,6 @@ extern NativeWindowFactory X11WindowFactory;
 #endif
 
 #ifdef SDL_VIDEO_DRIVER_COCOA
-/* Actually, we don't really do this, since it involves adding Objective C
-   support to the build system, which is a little tricky.  You can uncomment
-   it manually though and link testnativecocoa.m into the test application.
-*/
 #define TEST_NATIVE_COCOA
 extern NativeWindowFactory CocoaWindowFactory;
 #endif