Add SDL_GetTouchName

[Susko3]

SDL source: https://github.com/libsdl-org/SDL/blob/580d231822b20b52ce36dfc2918b691cadf5ac84/include/SDL_touch.h#L98-L104.

Uses same convention as SDL_JoystickName etc.:

SDL2-CS/src/SDL2.cs

Lines 6621 to 6631 in dd7503d

	/* joystick refers to an SDL_Joystick* */
	[DllImport(nativeLibName, EntryPoint = "SDL_JoystickName", CallingConvention = CallingConvention.Cdecl)]
	private static extern IntPtr INTERNAL_SDL_JoystickName(
	IntPtr joystick
	);
	public static string SDL_JoystickName(IntPtr joystick)
	{
	return UTF8_ToManaged(
	INTERNAL_SDL_JoystickName(joystick)
	);
	}