Windows GDK Support (#5830)

* Added GDK

* Simplfied checks in SDL_config_wingdk.h

* Added testgdk sample

* Added GDK readme

* Fixed error in merge of SDL_windows.h

* Additional GDK fixes

* OpenWatcom should not export _SDL_GDKGetTaskQueue

* Formatting fixes

* Moved initialization code into SDL_GDKRunApp

include/SDL_rwops.h

@@ -98,7 +98,7 @@ typedef struct SDL_RWops
         {
             void *asset;
         } androidio;
-#elif defined(__WIN32__)
+#elif defined(__WIN32__) || defined(__GDK__)
         struct
         {
             SDL_bool append;