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/testnativew32.c

@@ -14,6 +14,8 @@
 
 #ifdef TEST_NATIVE_WINDOWS
 
+#include <windows.h>
+
 static void *CreateWindowNative(int w, int h);
 static void DestroyWindowNative(void *window);