Usage:
call GUI_setLoadProg(percent) to set the bar
set percent to -1 to not show bar
set percent from 0~1.0 to show current load percentage
call GUI_toggle() as leaving and re-entering menu to clear the screen and come back to GUI
Default is for centered text, but code should be added to the menu to call GUI_centerText(bool flag) to specify centered (TRUE) or left-justified (FALSE) text.