[FIX] web_editor: Media dialog next btn reload page

opw-1910505

Before this commit, the next button in the media dialog widget reloaded
the page and opened only the iframe of the widget.

Now, the button only update the media dialog widget.

closes odoo/odoo#29981

addons/web_editor/static/src/js/widgets/widgets.js

@@ -1288,6 +1288,7 @@ var MediaDialog = Dialog.extend({
      * @private
      */
     _onPagerClick: function (ev) {
+        ev.preventDefault();
         this.active.goToPage(this.active.page + ($(ev.currentTarget).hasClass('previous') ? -1 : 1));
         this._updateControlPanel();
     },