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

VisualC-GDK/clean.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+find . -type f \( -name '*.user' -o -name '*.sdf' -o -name '*.ncb' -o -name '*.suo' \) -print -delete
+find . -type f \( -name '*.bmp' -o -name '*.wav' -o -name '*.dat' \) -print -delete
+find . -depth -type d \( -name Gaming.Desktop.x64 \) -exec rm -rv {} \;