Make alt_intro.php order modular and introduce postprocess hook.
I would like to propose a postprocess hook in the typo3/alt_intro.php file in the main() function.
Furthermore, to do anything meaningful in the hook I also propose making the content 'modular', in the sense that the content is put into hash with 'parts' and 'order.
The patch attached solves both of the above, and keeps the layout just as before (if there are no hook), thus keeping backwards compatibility.
(issue imported from #M7313)