Consistently make all form callbacks take $element by reference

Problem

• Some form callbacks return a copy of $element, some others alter &$element by reference.

Goal

• Consistently pass $element to Form API callbacks.

Proposed solution

1. Change all Form API callbacks to take $element by reference:

   D7:

   function form_process_radios($element) {
     ...
       $element['foo'] = 'bar';
     ...
     return $element;
   }
   

   D8:

   function form_process_radios(&$element) {
     ...
       $element['foo'] = 'bar';
     ...
   }
   

Notes

• If form elements will ever be objects, they will behave as if they were passed by reference, too. So this is a compatible step into that direction.
• Render API should ideally be changed to follow this consistency, since Form API elements usually interact deeply with Render API.