[FIX] web_editor: preserve note-editable class on editable in iframe

The "note-editable" class was lost on the editable element when loading
it into an iframe. As a result, some selectors failed, making it so that
some options wouldn't activate when they should.

closes odoo/odoo#69908

X-original-commit: d71b7d87daa5b33f16eb9c6ade29580fb2076313
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/web_editor/static/src/js/wysiwyg/wysiwyg_iframe.js

@@ -121,7 +121,7 @@ Wysiwyg.include({
 
                 const $iframeWrapper = $('<div class="iframe-editor-wrapper">');
                 const $codeview = $('<textarea class="o_codeview d-none"/>');
-                self.$editable.attr('class', 'o_editable oe_structure');
+                self.$editable.addClass('o_editable oe_structure');
 
                 $iframeTarget.append($iframeWrapper);
                 $iframeTarget.append($utilsZone);