[FIX] web_editor: properly allow to reset bg color

If resetting a background color was the only edition that was done on a
page and that this page was saved, the change was not properly saved
as the page was not marked as dirty.

Closes https://github.com/odoo/odoo/pull/27625
task-1879524

addons/web_editor/static/src/js/snippets.options.js

@@ -565,6 +565,7 @@ odoo.define('web_editor.snippets.options', function (require) {
 
             this.$el.find('.note-color-reset').on('click', function () {
                 self.$target.removeClass(self.classes);
+                self.$target.trigger('content_changed');
                 $colors.removeClass("selected");
             });
         }