Changed SDL_FlashWindow() so it doesn't take a flash count, and added the hint SDL_HINT_WINDOW_FLASH_COUNT to control behavior on Windows

include/SDL_hints.h

@@ -1440,6 +1440,15 @@ extern "C" {
  */
 #define SDL_HINT_WAVE_TRUNCATION   "SDL_WAVE_TRUNCATION"
 
+/**
+ *  \brief  Controls the number of times a window flashes with SDL_FlashWindow()
+ *
+ *  On Windows, if this variable is set, the SDL_FlashWindow() call will flash
+ *  the specified number of times. Otherwise the window will flash until it
+ *  becomes the foreground window.
+ */
+#define SDL_HINT_WINDOW_FLASH_COUNT "SDL_WINDOW_FLASH_COUNT"
+
 /**
  * \brief Tell SDL not to name threads on Windows with the 0x406D1388 Exception.
  *        The 0x406D1388 Exception is a trick used to inform Visual Studio of a