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_config.h

@@ -33,6 +33,8 @@
 #include "SDL_config_windows.h"
 #elif defined(__WINRT__)
 #include "SDL_config_winrt.h"
+#elif defined(__WINGDK__)
+#include "SDL_config_wingdk.h"
 #elif defined(__MACOSX__)
 #include "SDL_config_macosx.h"
 #elif defined(__IPHONEOS__)