WinRT: hiding SDL_WinRTRunApp's XAML functionality, until it's ready for use

SDL_WinRTRunApp() is used on WinRT to launch a main(int, char **)-style
function.  It has optional, and experimental support for launching content
inside a XAML control, backed by a main() function running on a separate thread.
This is provided via it's 2nd parameter, which can be a pointer to a XAML
control.  (If NULL, XAML support will not be used.)

This change renames the experimental feature's parameter (to SDL_WinRTRunApp())
as "reserved", until such time as the functionality is ready for use.  It will
likely be renamed again in the future, when running SDL on top of a XAML control
via a separate thread, becomes reasonably usable.