Allow lazy loading per-element CKEditor by element #property

If you have 10 fieldsets and 40 editors, you don't want all editors to init immediately, because that makes the page crazy slow. Instead, the editor should only load when the parent fieldset opens. Or in another very smart way that a developer should decide.

To allow for this, a specific CKEditor instance must be disabled by default. There is an option for this: 'autostart'. Autostart is populated in ckeditor_load_by_field() by $ckeditor_on per element, so that's perfect. BUT it can't be altered or preset. It comes from the CKEditor profile, which is global.

A very simple, completely backward compatible check would make this possible:

  // Allow per-element 'autostart'.
  if (isset($field['#ckeditor_on'])) {
    $ckeditor_on = $field['#ckeditor_on'];
  }

(It's called #ckeditor_on because that's the variable name and it includes "ckeditor", so it's better than #autostart.)

By disabled your 40 ckeditors, a developer could add this JS in the form to make them lazy load:

// Lazy loading CKEditors in fieldsets.
// Add listener to <form>, because event bubbles up from any <fieldset> to the <form>.
$('#domain-locale-conf-all-form', context).bind('collapsed', function(e) {
  if (!e.value) {
    var fs = this;
    setTimeout(function() {
      // Find all newly visible editors.
      var ids = $('textarea.ckeditor-mod:not(.ckeditor-processed):visible', fs).map(function(i, ta) {
        return ta.id;
      });
      if (ids.length) {
        console.debug('Lazy loading ' + ids.length + ' CKEditors.');

        // Need ckeditorToggle(), because ckeditorOn() won't change the toggle label.
        Drupal.ckeditorToggle(ids, Drupal.t('Switch to plain text editor'), Drupal.t('Switch to rich text editor'));
      }
    }, 1);
  }
});

I'll try to make a patch, but I use a severely hacked 1.16, so I'm not sure it'll apply or even match at all.